An analysis is made of the calibration principles and limitations for differential reflectivity （〖WTBX〗Z〖WTBZ〗dr） of dual polarization weather radar using external targets such as drizzle, sun, Bragg scattering at the top of the convective boundary layer and ground clutter. An engineering calibration method that can obtain the〖WTBX〗 Z〖WTBZ〗dr bias induced by transmitter, receiver, and antenna online is presented. In addition, a method is put forward to improve 〖WTBX〗Z〖WTBZ〗dr measurement accuracy, which compensates variations in transmitted power and receiver gain by measuring coupled transmitted power and backscattered power of meteorological targets at the output of digital receiver during each pulse repetition time in horizontal and vertical channels, respectively. These two engineering calibration methods are applicable to radar that simultaneously transmit and receive waves in horizontal and vertical polarization channels and can satisfy operational requirements for online calibration.