Abstract: Cooling system is very important for active phased array radar. This paper introduces the design and composition of the cooling system of typical active phased array radar, and analyzes the differences of cooling system structure and flow distribution method between the rectangular and the circular active phased array radar cooling system. The computation of the flow resistance of the cooling system concludes that in the case of neglecting the mainstream flow resistance, the square cooling system can carry out the flow distribution of the coolant by itself, while the circular one needs to match the flow resistance first in order to meet the requirement of the flow distribution. For the circular active phased array radar cooling system, a simple and feasible flow distribution method is introduced in this paper with an example.