Following the continuous increasing of power density of nowadays power converter, more and more attention is paid to thermal system design, since its performance is the key index to affect the power density and also determine the reliability of power converters. The rectangular fin-type air cooling heatsink is a common heatsink design in power converters. The modeling of such heatsinks become an important step for power converter design optimization. This paper proposes an analytical model for air-cooling system with multiple discrete device used in parallel. In this model, an air temperature rise model is constructed to reveal the air temperature distribution considering the position of the power devices and the flow direction of air in the heatsink channel. And a correction term with thermal diffusion angle is introduced to solve the mismatch problem between the size of the power device and the surface area of heatsink. Finally, the simulation results verify the validity of the proposed model and its accuracy.