Based on elastic-plastic contact theory, an improved algorithm is presented to calculate the parameters of the gear contact force, while considering variable conditions impact, meshing impact and pitch point impact. Meanwhile, the flexible multi-body gear transmission dynamic model is built with multi-body software and an improved algorithm. This model and algorithm can be applied to the analysis dynamics of the gear transmission with large contact deformation and frequent impact. Results show that (1) the dynamic meshing force that is caused by the variable conditions impact is greater than that caused by the meshing impact; (2) The meshing impact causes the different periodic fluctuation of the dynamic meshing force at different loading cases; (3) The sliding friction factor has great impact on the gear tangential friction pitch point impact. The results are significant for the calculation of dynamic contact force parameters and the analysis of gear transmission transient dynamics.