The fluctuation of outlet flow of reciprocatingtype positive displacement pump with multi plunger and check valve is determined by the structural characteristics and operational principle of this kind of pump, which is the inherent property of this kind of pump. In this study, the theoretical model of dimensionless instantaneous outlet of 3-plunger and 5-plunger reciprocating positive displacement pump were established without considering the dynamic process of valve and leakage effect of plunger pair, and the cycle working rhythm and flow fluctuation mechanism of these two kinds of pump were analyzed; the upper and lower flow fluctuation rates were introduced to quantitatively measure the flow fluctuation, and the influence of the number of plunger and the ratio of crankshaft rotation radius to connecting rod length on the flow fluctuation rate was analyzed; the hydraulic simulation model of 3-plunger pump without accumulator was built based on AMESim, and a comparative study with the theoretical analysis results was carried out on the base of dimensionless process of the simulation data. The results show that: there are two peaks and one trough in the outlet flow curve among every working cycle of this kind of pump, which results in smaller upper flow fluctuation rate and bigger lower flow fluctuation rate; for odd number plunger pump, the flow fluctuation rate increases with the increase of the ratio of crankshaft rotation radius to connecting rod length; for even number plunger pump, the flow fluctuation rate is higher and independent of this ratio; the inlet valve closing lag of this kind of pump causes the position lag and depth deepening of the flow curve trough, and leads to the position of the two peaks in the same cycle is no longer symmetrical and the height is no longer equal.The results of this study can be used as reference for the improvement design of this kind of pump with variable displacement and the research of fluid supply with steady pressure technology oriented to the action process of hydraulic loading.