The basic idea of Software Defined Radio is to structure a general hardware platform to make the most of the software to complete variety of communication functions. As one of the key technology of Software Defined Radio, Digital Upconverter modulated the input data in kinds of modes and converted it to intermediate frequency, It is difficult to implement it on Digital Signal Processor, so we should use special IC to achieve DUC. AD9857 is a Quadrature Digital Upconverter chip manufactured by ADI corporation with features such as programmable, small package, high speed and high performance. First, the authors introduce the principle, architecture and application of AD9857, and then analyse the characteristic of Software Defined Radio modulation, at last they give an instance in which they design a plat of general digital modulation system integrated FPGA and AD9857.