An aperture synthesis passive millimeter wave (PMMW) imager using 256 antennas with 1 GHz bandwidth is currently under development in Beihang University. This imager is designed for concealed contraband detection on human body in indoor environment at video imaging rate. Design of the digital cross-correlation subsystem is presented in this paper. High-speed clocked comparators are used to perform 1-bit A/D convention, while FPGAs are used to perform further signal processing. The cross-correlation system is split into three parts: 1-bit A/D conversion, data distribution and signal processing. The analog signals from receivers are sampled and distributed to several cross-correlation processors based on FPGA. A sampling scheme performing per-bit deskew and serial-to-parallel conversion and a correlator performing cross-correlation of 128 channels on a signal FPGA are presented in this paper. Test results from a 64-channel test board are also presented, which prove the design works.