Quick start has become an inevitable requirement for pumps in many application fields. However, abnormal transient pressure and pulsation characteristics can cause start-up failures, and even safety accidents. In this study, a measurement system was established for the transient external characteristics and pressure pulsation during the start-up of mixed flow pump, in order to clarify the transient starting law of mixed flow pump. An investigation was made to explore the time-frequency characteristics of transient pressure pulsation under various start-up acceleration and flow rate, thereby analyzing the time-frequency evolution of starting process. A frequency converter was used to set the start-up time. The LWGY-250 turbine flowmeter and MPM480 pressure sensor were used to measure the transient flow and transient pressure, respectively. The wavelet transform was selected to analyze the non-stationary pressure pulsation signal during the start-up process. The results showed that the flow and head curves were approximately divided into 3 stages: a slow rise, a rapid rise, and slow flat, as the rotating speed increased during the start-up process, where the duration of 3 stages was negatively correlated with the start-up acceleration. The head curve dropped slowly and stabilized after a rapid rise until the impact head. In addition, the increase of starting acceleration had a positive stimulating effect on the external characteristics of speed, flow, and head. The pressure shock was normally caused by the angular acceleration and low-condition operation, which were related to the flow rate and start-up acceleration at the moment of the end of start-up. There was more significant pressure shock as the starting time decreased when the flow rate was constant. The inlet pressure shock weakened gradually or even disappeared, but there was a more obvious pressure shock between the middle of impeller and the outlet, as the flow rate increased, when the starting time was fixed. When the start-up time was 1 s and the stable working condition was 1.2Q, the maximum impact pressure at the middle of the impeller and the outlet can reach 37 and 28 kPa, respectively. In high start-up acceleration, the flow rate had no effect on the transient pressure change during start-up process, whereas, there was an obvious effect of flow rate on the pressure change after the end of start-up. The pressure difference between 2 types of flow rate increased in the middle of impeller reached 30 kPa when the pressure tended to be stable. In low start-up acceleration, the effect of flow rate on the starting characteristics was significantly reflected in the start-up process. The pressure difference between the middle and the outlet of impeller reached 15 kPa at the end of the start. The main frequency of impeller was the blade frequency and its multiplier during the start-up process. Moreover, the evolution trend of main frequency was consistent with the changing trend of rotating speed. There was a maximum amplitude of main frequency caused by the pressure shock at the end of the start under the fast start condition. The findings can provide sound references to reveal the transient operating characteristics for the optimization design of transient working pumps.