This article describes a method of IF modulation used in millimeter-wave transmitter, the method is AD9857 combined with FPGA, choose the EP1C6 as a FPGA chip, and use this FPGA chip complete the base-band digital signal processing, and implement the control to the AD9857. AD9857 is a kind of the orthogonal frequency converter based on DDS technology, complete the functions of base-band digital signal interpolation filter, orthogonal modulation, and D/A conversion in the digital domain and complete QDPSK modulation signal output of 70MHz IF carrier. This method can eliminate phase and gain imbalances and distortion which caused by analog modulation, also can simplify the system structure, reduce cost and improve the performance and reliability of the system.