Discharge monitor is a kind of primary payload monitor loaded on the manned spacecraft in the Low Earth Orbit, which is designed for monitoring the secondary discharge problem on the solar array. The issue of integrated electronic device heat dissipation is the key for normal operation supporting. In this study, the integral payload is simulated by finite element method, through structure modeling, based on the theory of heat conduction and heat radiation. In the result, under the measures of adjusting the layout of the electronic devices, coating the circuit board with copper, and black anodization for the case, the payload could definitely meet the needs of the heat dissipation in the extreme conditions, survive safely and functionalize perfectly in the orbit, at least from the angle of thermal design.