In the investigation,the structure features and start-up characteristics of vertical mixed-flow pumps areanalyzed.Then,with the maximum water hammer pressure in the start-up process as the control target,an orthogo-nal test table is designed to calculate the 3-D water hammer and analyze the influences of the electric motor start-uptime,the butterfly valve opening time and the blade angle on the pump start-up process,which provides a strongtheoretical basis for the start-up operation optimization of the vertical mixed-flow pump.The results show that (1)as the electric motor start-up time increases,the maximum pressure of the pump start-up water hammer decreasesgradually,with a maximum amplitude of 4.4%; (2) as the butterfly valve opening time increases,the maximumpressure increases gradually,with a maximum amplitude of 3.4%; (3) with the increase of the blade angle,thechange of the maximum water pressure becomes more complicated; and (4) the effect of the electric motor start-uptime on the maximum water hammer pressure is the most significant while the effect of the butterfly valve openingtime is the weakest.Besides,by taking into account the fact that there exist saddle regions in the head curves atcertain blade angles and from the perspective of guaranteeing the normal running of water pump,the optimal start-up parameters are found to be an electric motor start-up time of 26s,a butterfly valve open time of 30s and a bladeangle of －3 °.