Abstract: Aiming at the problem that the load capacity of forced guided oil circulation air-cooled(ODAF) transformer is affected by the temperature rise constraint, this paper proposes three kinds of load capacity evaluation methods for transformer. Firstly, the running state of fan and oil pump and the influence of oil viscosity change on thermal resistance and other factors are taken into account, and a transformer thermal model is established based on thermoelectric analogy method to calculate winding hot spot and top oil temperature. Secondly, particle swarm optimization algorithm(PSO) is used to fit the parameters of the thermal model; meanwhile, based on the operation data of two different types of transformers, the calculation accuracy of the thermal model and the suitability of the fitting parameters are validated. Finally, a load capacity evaluation model based on the temperature rise characteristics is proposed by referring to the temperature rise limit given in the load guide GB/T 1094.7. The analysis results show that the error of hot spot temperature calculated by the proposed model is no more than 2.35 ℃, which is within the allowable range of engineering. Under normal periodic load, when the ambient temperature is lower than 1 ℃, the temperature rise constraint is still satisfied after closing a group of sub-radiators.