Gallium Nitride (GaN) power devices are conducive to the development of high-frequency and high-efficiency aerospace power supply. However, the inevitable parasitic parameters in the circuit are easy to cause oscillation and lead to the deterioration of circuit reliability. An active clamp forward DC/DC converter with parasitic parameters is proposed, using GaN power device as the main switch to improve the switching frequency and efficiency of the power supply. The main operating modes of the circuit are deduced theoretically, and the topology of the circuit containing three kinds of parasitic inductors is analyzed dynamically by simulation. Finally, according to the simulation results, the optimization measures for the parasitic parameters causing different oscillation problems are proposed. An experimental prototype is designed and experimentally tested, proving the effectiveness of the suppression methods. Promote the high-reliability application of GaN power devices in aerospace power supply.