Abstract The crankshaft is the key moving part and stressed part of the reciprocating pump. The crankshaft is composed of multiple eccentric shafts, which is not conducive to avoid stress concentration due to structural mutation. Because the crankshaft bears the combined bending and torsion stress, it is very difficult to calculate accurately. Taking the crankshaft calculation of a five-cylinder reciprocating pump as an example, ADAMS software was used to calculate the flexible body dynamics of the crankshaft system, and obtain the motion law and stress situation of the crankshaft in four cycles. The results show that the chamfering stress of the five cranks is large, and the accurate extreme stress point and corresponding position are found out. The maximum value of the chamfer stress on the left side of crank 2 is 93.23 MPa, which appears periodically at the four angular positions of 216°, 575°, 936° and 1 296°. The results are verified by statics. The maximum stress value of statics calculation is 92.46 MPa, with a deviation of 0.8%. The calculation results are basically consistent with the actual work of the crankshaft, which provides a reference for the design and further optimization of the crankshaft, and is of certain guiding significance to engineering design.