The performance of a rotary damper which can control the falling speed of the cover plate depends on its internal cylindrical cam mechanism,spring and viscous damper.In order to calculate the performance of the rotary damper system,the transmission action of the cam structure and the output characteristics of spring and viscous damper were analyzed; a theoretical calculation dynamic model of the falling condition of rotary damper system was created and calculated by using Runge-Kutta method; the model was used to analyze the parameter matching of the viscous damper; the accuracy of the calculation model was verified through experiment.The results show that the model can be used for parameter matching design of the rotary damper.