Abstract: The calculation of transient heating load is fundamental to the design of the environmental control system (ECS) for avionics pods. In this paper the dynamic heat-balance equation for the avionics pod and the heat transfer equation for the pod wall have been established. The transient heating load of the pod have been computed by means of finite difference equations. The influences of the flight condition and the thermal inertia of the pod on the transient temperature in the pod have been analysed. Finally the paper has adopted the design concept of cool storage for saving energy and rationally defined the design heating load of the ECS for the avionics pod.