We analyze the channel mismatches in a multi-channel demultiplexing photonic analog-to-digital converter (PADC) applied in wideband radar receiving. We demonstrate a model of the channel mismatches and a method for extracting the mismatch information, and develop a channel-mismatch compensation algorithm. We analyze and compare the short-time Fourier transform spectra of the echo signals from the corner reflectors and the drone before and after the channel-mismatch compensation. The results confirm that the compensation algorithm can effectively suppress the intrinsic mismatches in the amplitude and delay mismatch caused by the multi-channel demultiplexing architecture of the PADC. The radar achieves wideband detection, covering a 4 GHz-bandwidth segment of the X-band and Ka-band signals. By comparison between the target detections of static and long-distance flying corner reflectors, we find that the compensated PADC effectively performs one-dimensional imaging of the corner reflectors installed 10 cm apart.