The dynamic characteristic simulation analysis model of a directional drilling machine power head gearbox for different levels of complex is established based on the detailed introduction of the modeling method of main components in gearbox,the influence of the stiffness of bearings and housing on dynamic characteristic of gear drive system is discussed. The results show that the bearings stiffness and housing flexibility has a significant influence on the dynamic characteristic and cannot be ignored in the analytical model; Increase the bearing stiffness and housing stiffness can effectively improve the system natural frequency,reduce the modal displacement and the gear dynamic transmission error; The gear tooth surface contact stress is relatively sensitive to the gearbox housing,but is not to bearing stiffness. The modeling presented method and the results of simulation on dynamic performance of the gearbox has an important guiding significance to the design and analysis.