Many factors, such as, dynamic characteristics of hydro-cylinder, structural flexibility, excavating objects and excavator-soil interaction affect digging dynamic behavior of hydraulic excavators during their operation. Here, to solve digging dynamic load calculation problems, an excavator’s digging dynamic model comprehensively considering factors mentioned above was proposed. To achieve a good balance between accuracy and efficiency, all slender structures including movable arms, bucket rods and connecting rods were simplified as beam elements. Complex shape structures like bucket, etc. were simply modeled using the sub-structure DOF condensation method. The bucket-excavating object relation was modeled as a spring-damper element. Hydro-cylinders were modeled as two-node elements containing variables of pressure, displacement and velocity, etc. Then, all structural elements and hydro-cylinder elements were assembled to form a global finite element system model, and perform dynamic computation using Newmark algorithm. Finally, a 50t excavator’s movable arm lifting under impact condition was taken as a case study to do computation and test analysis. The results showed that the computed dynamic loads agree well with measured ones in tests, the error of dynamic load peak values is less than 6%, these verify the correctness of the proposed method and model; compared with using the displacement driven model to simulate hydro-cylinder in dynamic computation, the accuracy of applying the proposed hydro-cylinder model in dynamic computation is increased by 5 times, while structural flexibility has a smaller effect on digging dynamic load. 