A technique of online calibration based on SYSCAL for CINRAD weather radar adopts is introduced, which can dynamically test and adjust the measurement errors for the performance variation of transmitting and receiving channels in a running radar, and the calibration has a good performance and an appropriate time interval. A detailed analysis is presented by the theoretical deduction of the online calibration technique of CINRAD weather radar reflectivity factor. The realization methods of the technique and corresponding calibration methods and its derivation in CINRAD/SA weather radar are described. The actual effect is tested and analyzed with a calibration example of the Wuhan weather radar. Some problems of the online calibration are listed. To master the principles and the technique of the online calibration is useful for correctly implementing calibration, ensuring the echo quality and improving the ability to diagnose the radar faults.