Based on the method of gear system dynamics and Lagrange equation, a translational-rotational coupling dynamic model of the herringbone planetary gear sets is built and dynamic analysis is carried out. The analysis results have guiding meanings for the design of herringbone planetary gear transmission system. Herringbone gear is treated as two helical gears in the model, and gear train elastic coupling, load inertia and stagger angle between the left and right helical gears is considered. The load sharing coefficients of the system are obtained through calculation, and the impact of stagger angle and coupling torsional stiffness between the left and the right helical gears on the load sharing characteristic is analyzed. The analysis results show that when stagger angle is π/2, the load sharing coefficients are the best, and coupling torsional stiffness between the left and the right helical gears has litter influence on the load sharing coefficients.