The phenomenon and the characteristics of a comprehensive failure of CINRAD/SB radar at Changde are described. The causal analysis of failure indicates that the diagnosis and maintenance of weather radar need to be conducted in a comprehensive manner, such as using indicator lamp, breakdown alert, and radar performance data, as well as combining software with hardware methods. Meanwhile the understanding of the three calibration principles and their signal flows relevant to breakdowns can help analyze, isolate, and locate breakdowns more effectively and accurately.