This letter proposes a self-calibration method for an active uniform linear array (ULA) of a multibeam surveillance radar. The approach utilizes the radar returns containing the clutter and the opportunity targets, which requires no additional hardware and time for the calibration. The array error is estimated from the phase gradient of the active ULAs and the Fourier property of ULA beamforming. To correct the estimation error, several range bins used as the estimation sources are selected according to an entropy value. The linear phase component in the phase error is eliminated using both the center shifting and the matched filter, thus, enhancing the calibration performance. The effectiveness of the proposed method against that of conventional ones is demonstrated with a computer simulation and a field experiment.