Aiming at the cycloid transmission in the cycloid planetary gear speed reducer used for robot,the analysis model for return clearance and mesh force calculation of cycloid transmission is set based on gear meshing theory and dynamic multi teeth meshing property. The influence of different tooth modification methods on the number of meshing teeth,force and distortion of each mesh point and the return clearance of cycloid transmission is calculated through iterative method. Furthermore,the influence of the load on the meshing characteristic is analyzed,and the stiffness curve of the cycloid transmission is obtained with the increase of load. It provides basis for the design and torsional vibration analysis of cycloid transmission.