Abstract: Internal flow has a significant impact on the performance and stability of a mixed-flow pump during deceleration. This article aims to explore the complex internal flow in mixed-flow pumps during deceleration. A multi-channel testing system was constructed to clarify the impact of different decelerations on the pressure pulsation inside the pump. External characteristic parameters and internal pressure pulsation of mixed-flow pumps were collected under different deceleration modes. Non-stationary signal processing was introduced to determine the effects of speed reduction on the pressure pulsation at the impeller outlet and inside the guide vanes. Three domains were divided into time, frequency, and time-frequency, as well as the correlation between them. The results show that the decline gradient of head and flow under different deceleration modes was positively correlated with the decline speed. The energy entropy ratio of the pressure pulsation in the pump showed a downward trend during deceleration. It indicated that the pressure pulsation was gradually stabilized with the decrease of the speed in the pump. The peak value of pressure pulsation and the changing trend of sample entropy were positively correlated with the exponential of the deceleration modes. The changing trend of impeller outlet pressure pulsation was consistent with the speed transformation during the whole deceleration. The forward transfer entropy under different deceleration modes was greater than the reverse transfer entropy. Therefore, the pressure pulsation at the outlet of the impeller was dominated that in the guide vane. The high energy region of the pressure pulsation was mainly concentrated in the blade passing frequency 4fn and the high-order harmonic 8fn, whereas, the amplitude was positively correlated with the index of the speed transformation. The strong correlation region was concentrated in the two frequency bands of rotation frequency fn and blade passing frequency 4fn in the process of deceleration. The high amplitude region of pressure pulsation at the outlet of the impeller was concentrated in the blade passing frequency of 4fn, while the frequency of the pressure fluctuation component with the highest amplitude inside the guide vane was 3fn. The partial coherence frequency between the pressure pulsation at the impeller outlet and inside the guide vane was in the range of 80 and 120 Hz. There was no change in the frequency range of the strong correlation region between the pressure pulsations at the impeller outlet and inside the guide vane during different decelerations. But there was a variation in the proportion of the strong correlation region in the frequency range of the entire deceleration. In the deceleration with the fixed initial speed and target speed, the deceleration mode shared no effect on the coherent frequency of pressure fluctuation in the impeller outlet and guide vane. The finding can provide a strong reference to reveal the transient operation for the high stability of mixed-flow pump in down-speed operation.