To study the dynamic characteristics of the gear engagement in the rotating system of the segment assembling machine, the transmission system of the pinion and the big gear ring of the segment assembling machine is taken as the research object. First, a six-degree-of-freedom spur gear dynamic model considering the friction of its tooth surface is established based on the lumped parameter method. Then, the rotating system of the segment assembling machine is analyzed by the Runge-Kutta method based on the parameters of the segment assembling machine in real working conditions. Finally, the vibration of the rotating system of the segment assembling machine is tested to verify the correctness of the model and the solution. Another method for reducing the vibration of the segment assembling machine is designed by increasing the rigidity of the rotating system. The research is valuable to the vibration reduction and noise reduction treatment of the rotary system of the segment assembling machine and the improvement of the accuracy of the rotary system of the segment assembling machine.