With the introduction of concepts, such as clean power plants and smart power plants, higher demands are placed on the operating environment of power plants, the reliability and longevity of hydro generator set operation. The carbon brush/slip ring assembly is an important part of the excitation system of a hydro generator set, and the excitation current is transmitted through sliding contact between the two. In actual operation, the carbon brush/slip ring assembly often has problems, such as carbon brush overheating, ignition and arc erosion due to the change of interface contact state. In this way, it not only causes temporary elimination of the hydro generator set, but also seriously affects the safety and reliability of hydro generator set operation. Since most online monitoring systems can only monitor the temperature and overcurrent of carbon brushes and wear parameters, they cannot directly monitor the contact state between carbon brush/slip ring assembly, so indirect monitoring becomes the key to solve the problem. The contact state between carbon brush/sip ring assembly can be characterized by friction coefficient, contact resistance, contact voltage drop, etc. For current-carrying friction systems, the most commonly used parameter is the contact resistance between the interfaces. The contact resistance can not only reflect the rough contact state between the interfaces and the generation of surface film, but also to some extent the distance between the friction interfaces. Generally speaking, the rougher the surface, the greater the distance between the two friction surfaces, the smaller the effective conductive area between the friction sub, the greater the contact resistance. When the contact resistance between the friction subsets increases, the temperature of the carbon brush/slip ring assembly will further increase. If the relationship between temperature and contact resistance can be established, the contact state can be indirectly reflected by temperature. Therefore, this paper established a mathematical model of contact resistance based on the theories related to electrical contact and heat transfer. The model aimed to estimate the contact resistance value based on the carbon brush temperature. The relevant parameters in the model were determined by the carbon brush/slip ring current-carrying friction test, and the dynamic changes of contact resistance and the key influencing factors were calculated and analyzed under different rated current, rotational speed and spring pressure conditions. The results of the calculation were substituted into COMSOL Multiphysics finite element analysis software, and the temperature rise of the carbon brush was simulated and compared with the temperature rise of the carbon brush obtained from the test based on its built-in physical field control equations to verify the validity of the model. The calculation results showed that the contact resistance peaked at the beginning of the system operation, and then levelled off gradually. When the interface contact pressure was certain, the contact resistance value decreased with the increase of rated current; under the condition of certain rated current, the contact resistance value of carbon brush with high interface contact pressure was small. When the rotational speed of collector ring increased, the influence of spring pressure on the contact resistance between friction pairs further increased. When the rated current reached above 1 000 A, the contact resistance between the friction interface further increased with the increase of slip ring rotational speed.