The good design of BIT (Built in Test) and Fault diagnosis is an important way to achieve the better testability and maintainability of Phased Array Radar. This paper presents the basic idea of BIT design of radar system. Aim at the composition and function of a multi-functional Phased Array Radar, This paper designed a distributed BIT structure combining centralized control and hierarchical detection and the corresponding BIT workflow, and the selection basis of different fault diagnosis methods. Through the experimental verification based on fault injection, the fault detection rate of the radar BIT system is greater than 95%, the fault isolation rate is greater than 90% in the case of isolation to 3 LRU, greater than 85% in the case of isolation to 1 LRU, and the fault false alarm rate is greater than 5%, which meets the overall requirements and proves the effectiveness of the design of the BIT. At last, It looks forward to the improvement direction of on-line testing and fault diagnosis technology in the Phased Array Radar system.