Abstract: Mixed-flow pumps are characterized by their wide operating range and high-efficiency region in the fields of agricultural irrigation, hydropower engineering, and urban water supply systems. The stability of the transient processes, such as start-up and shutdown, has drawn great attention to the increasing capacity and impeller diameter of mixed-flow pumps in the industrial field. Particularly, the start-up process of a mixed-flow pump is an extremely complex transient process. The mixed-flow pumps can experience shock loading, hydraulic vibrations, and cavitation damage during the start-up process, leading to a negative impact on the stable operation of the pump. The evolution of noise characteristics at the pump section is of great significance for stability monitoring during the start-up process of mixed-flow pumps. This study aims to investigate the variation and influencing factors of noise under different start-up modes. An acoustic vibration test system was selected to collect the noise, shaft vibration, and pressure fluctuation signal of mixed-flow pumps under linear and non-linear starting modes. Then, the collected noise signals were analyzed using acoustic signal processing, including the energy entropy ratio analysis, spectral chart analysis, and A-weighted sound pressure calculation. A systematic analysis was made to clarify the influence of start-up on the sound pressure level and spectral characteristics of noise. The energy intensity in the noise was also analyzed. A coherence analysis was then implemented to determine the correlation among the impeller outlet pressure fluctuation, shaft vibration, and pump noise during the start-up process. The results showed that there was a concentrated region in the high amplitude sound pressure of the noise starting at the end of the start-up process in the three start-up modes. The shortest time scale was achieved in the high amplitude noise region under the concave exponential function start-up mode, indicating the least persistent impact on the environment. There was a relatively minimal impact noise generated by the concave exponential function start-up mode, indicating that this start-up mode was beneficial to avoid the severe mechanical impact during start-up. In addition, the formant analysis of the noise spectrogram showed that there was the lowest probability of mixed-flow pump’s instability induced by the concave exponential function start-up mode among the three start-up modes. The highest A-weighted sound pressure level of the noise generated by the pump was located at the frequency band with the center frequency of 250 Hz. The maximum A-weighted sound pressure level and the total effective sound pressure level of the noise generated by the pump under the linear start-up mode were less than those of the other two start-up modes. Compared with the linear start-up mode, the concave exponential function start-up mode improved the sound pressure level of the medium- and low-frequency noise. The wavelet partial coherence analysis showed that the impeller outlet pressure fluctuation was the main factor affecting the dominant sound pressure level (center frequency was 250 Hz) of the noise during the start-up process. The radial vibration of the shaft only affected the noise at the very low-frequency band.