To solve the systematic error estimation problem for a two-coordinate radar, the least squares radar systematic error estimation method by a joint ADS-B (Automatic Dependent Surveillance-Broadcast) device is proposed. In the proposed method, the ADS-B measurements are first transformed from geographic coordinate system to radar local rectangular coordinate system for building a unified observation space. Then, the radar track and ADS-B track are applied to calculate the corresponding straight-line equation by the line fitting algorithm, and these two equations are further utilized to calculate the angly between the two straight-lines for compenlating the azimuth component of the radar track. Finally, the least squares algorithm is applied to estimate the radar systematic error. The real-data experimental results of real data illustrate that the proposed method can effectively estimate the radar systematic error compared with the traditional straight-line algorithm and the least squares algorithm. After registering the radar track by the proposed method, the average range error and the azimuth error can be reduced by 71.7% and 52.7%, respectively.