Based on the theory and research on a GPS Wide-Area Augment System (WAAS) built in China. The composition and the basic theory of WAAS are analyzed. The shortcomings in traditional differential augment system are researched. According to the experience drawn from the past researches and theories, the theory and the arithmetic model related to ionosphere delay are provided, including a way of improving the precision of ionosphere delay in one-reference station. As a result, the arithmetic model is used to fully separate and estimate the ionosphere delay and frequency offset. In the end, the analysis, simulation and processing of the real data have proved that the model is reasonable, with having achieved the expected result