The thermal fault diagnosis of automatic production machinery and equipment and the analysis of its reliability are of positive meanings for promoting the advancement of diagnosis technology and guaranteeing the safe and reliable operation of the equipment. For this reason, this paper attempts to research the thermal fault diagnosis of transmission system in automatic production machinery and equipment and analyze its reliability. At first, with a transmission device as an example, thermal fault diagnosis was performed on the device and its reliability was analyzed; then, the paper gave the overall structure of the thermal fault diagnosis system and elaborated on the diagnosis flow. After that, the distribution of water temperature in radiator water channels was described in detail, and a temperature model of the transmission device were constructed. At last, this paper fully considered the possible interference factors in actual production, and used 3 kinds of parameter models for systematic identification, and the experimental results proved the effectiveness of the proposed model.