Abstract: The inlet structures of pipeline pump are characterized by the elbow bend pipe. The inlet structures are susceptible to inlet reflux and uneven impeller inflow, leading to substantial hydraulic losses. This study aims to investigates the flow loss characteristics during the operation of pipeline pump using numerical simulation method. Computational fluid dynamics (CFD) was employed to predict the energy characteristics and internal flow patterns. An opening test bench was established to measure the performance parameters of the pipeline pump. A comparison of performance parameters between numerical results and experimental data was conducted to validate the reliability of numerical simulation method. The entropy production theory was also used to qualitatively and quantitatively analyze the magnitude, and identify the specific location of flow loss. A system analysis was made to recognizing the significant impact of suction conditions on the impeller inlet, and the flow state inside the impeller dominates the energy conversion efficiency of the entire pipeline pump. Therefore, the entropy production of the suction and impeller was discussed emphatically, and explore to construct the relationship between the flow loss and the flow field. The results indicate that the energy curve obtained by numerical simulation method was better consistent with the experimental results data. The numerical results data was much little higher than the experimental results ones, but the error between the two was less than 5%, indicating that high credibility of the employed numerical simulation method. The total entropy production of the suction and impeller exhibited a trend of initially decreasing and then increasing with rising flow rate. This behavior was aligned closely with the variation in the main frequency amplitude of pressure fluctuation observed at internal monitoring points. Notably, the total entropy production was smaller in the high-efficiency region and, whereas, the larger in the low-efficiency region, indicating that the entropy production was better represented on the flow loss of the pipeline pump. Furthermore, the variation in total entropy production of the impeller was highly consistent with the suction, indicating that the flow inside the suction affected the flow inside the impeller. The total entropy production of the impeller was outstandingly greater than that of the suction, especially that under biased flow conditions. The total entropy production of the impeller was about four times that of the suction at low flow rate, while the total entropy production of the impeller increased to approximately 15 times that of the suction at high flow rate. The zone of high entropy production rate zone in the suction was primarily distributed in the second bend and outlet, with the increase of flow rate, the entropy production rate showed a trend of first decreasing and then increasing. Notably, the entropy production rate under low flow conditions was hundreds of times higher than that under the rest conditions. The high entropy production rate observed in the suction region, particularly at the second bend and exit positions, can be attributed to the presence of large-scale nucleated vortices during low flow rates and wall-attached vortices during high flow rates. The high entropy production area of the impeller was primarily concentrated at the inlet and outlet. In the radial direction, under low flow rate conditions, the entropy production rate at the outlet was about 7 times higher than that at the inlet under the low flow rate. In the axial direction, the entropy production rate under low flow rate conditions was at least 6 times that under the rest conditions. However, there was no significant variation in the entropy production rate under other flow conditions along the radial and axial direction of the impeller. The high entropy production rate at the low flow rates was primarily caused by the pre-swirl at the impeller inlet and the large-scale stall vortex at the outlet. Conversely, at high flow rates, the high entropy production rate was driven by the severe wall detachment of the fluid and the formation of a strong separation vortex zone at high flow rates. This study finding can offer the valuable insights to identify and optimize the localized flow losses in pipeline pump and enable targeted optimization efforts.