Vertical detection calibration is an effective way to calibrate the differential reflectivity (〖WTBX〗Z〖WTBZ〗DR) of dualpolarization radar, it can correct measurement errors of 〖WTBX〗Z〖WTBZ〗DR, and improve the quality of the radar data. Based on the simultaneous transmission and reception mode of the Xband dualpolarization radar system, the ZDR product of the Xband dual polarization radar system has a large deviation in some areas, and it can preliminarily concluded that such problem is caused by the azimuth rotary joint by analyzing the radar hardware system and data of the vertical detection calibration. After replacing the damaged with new azimuth rotary joint, this problem can be solved well, which can be verified by the comparing of the data before and after this replacement. Several tests show that the data of vertical detection calibration can correct the ZDR offset effectively when using in the condition of mediumscale stratiform precipitation with the echo intensity between 255 dB and 305 dB.