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Strong pressure fluctuation existed in semi-open centrifugal screw pump with single blade under the combined influence of asymmetric impeller structure, static and dynamic coupling effects between impeller and volute and tip clearance leakage etc., which was not conducive to the safety operation of the units. The semiopen screw centrifugal pump with blade and specific speed of 237 was taken as research object, STAR—CCM+ simulation platform was used, calculation area was partitioned with polyhedral mesh, the steady and unsteady numerical calculations of whole flow field were conducted, meanwhile, the outer characteristic was predicted and the pressure fluctuation of inlet and middle area of the blade was monitored. Results showed that the error between predicted outer characteristic and test results was very small and the trend of monitored pressure fluctuation was basically consistent with the test, but when there was a distance between monitoring points and suction surface and a large circumferential distance from the same axial height of pressure surface, the results of numerical calculation were different from the test results. It can be found from the pressure time-domain diagram of two monitoring points that pressure fluctuation span of Pre2 was larger than that of Pre1, which was due to the work capacity of blade’s middle area was better than that of inlet area. Analysis of pressure contour of monitoring points showed that when the pressure surface was close to the monitoring point, pressure reached its maximum value, and at 1/3 circumferential thickness area of the blade, pressure reached its minimum value. In addition, in the range of the lowest point to the highest point, the pressure fluctuation of two monitoring points presented three kinds of slope change under five working conditions.