A bond graph model was established for the air-flow calculation of the diesel engine's heat dissipation system, to find out the relations between the design parameters and system's heat dissipation performance, especially when there was a lack of supportive data. By overall consideration of influences of the fan's parameters and the aerodynamics on the system's final performance, the bond graph was applied to calculate the power-loss during the process. Then a mathematic model of power flow with the design parameters of geometric parameters and air characteristics, where the flow-rate of the radiator's air duck could be calculated according to the fan's rotational speed. Formula of air speed distribution was also proposed that matched the data from field test. The bond graph model of the fan-channel is proved to be an effective estimation method of flow rate.