This paper takes an integrated rotary joint of cycloidal precision planetary transmission motor reducer as the research object, and studies the influence law of thermal effects on the stiffness of robot joints caused by the thermal effect jointly generated by motor power loss and heating and the friction and wear of meshing pairs. First, taking into account the thermoelastic coupling deformation of parts and the temperature gradient change of the material’s elastic modulus, combined with the existing cycloidal precision planetary transmission stiffness theory for analysis, the results show that under the working condition of 20 °C to 60 °C, the thermal effect has a relatively small impact on the contact stiffness and elastic torsional stiffness of cycloidal precision planetary transmission. Further, the thermal stress and thermoelastic coupling stress of the meshing pairs are introduced and calculated, and a thermal stiffness analysis model of cycloidal precision planetary transmission is established and analyzed. The results show that with the increase of the temperature gradient, the thermal stiffness of the meshing pairs gradually decreases, and with the increase of the load gradient, the thermal stiffness of the meshing pairs increases significantly, indicating that the thermal stiffness analysis model is very sensitive to changes in temperature and load and has potential engineering practical significance.