To obtain the optimal design method of the Inductance and Capacitance of output intrinsic safety Buck DCDC converter, the Output Short-Circuit Discharged Energy (OSCDE) of a buck DC-DC converter in Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM) are analyzed, respectively. It is pointed out that the energy in DCM is independent of inductance, while that in CCM increases with the inductance. The Maximum OSCDE (MOSCDE) within the whole operating range is deduced. If the MOSCDE is less than the corresponding minimum ignition energy, the converter meets the requirement of output intrinsic safety. Taking the derivatives with respect to inductance on the MOSCDE and letting the results to be zero, the optimal values of inductance and capacitance are obtained. Experiment results are in positive to the analysis showing the feasibility of the proposed methods.