Abstract: One of the difficulties in spaceborne electronic equipment's thermal design is heatspreader structure design for these chips whose heat sinks are on the top. Using a whole aluminium or copper heat-conducted plane to press on chips for heatspreader is a frequently used method, which would add unwanted weight and falls short of the need of lightening satellitic structure. This paper designed a cap-heatspreader structure. By flotherm software, heatspreader effect was simulated and dimensions were optimized. The result proved the cap-heatspreader structure could cool the chips effectively. Comparing with the whole metal plane, the cap-heatspreader structure could reduce weight effectively.