Abstract: In this paper, we analyze the COMPASS's structure firstly. Then we simulate and analyze the system's visibility, DOP value and position accuracy in China Mainland. Based on it, according to the features of the system's constellation and referring the principle of Space-Based Augmentation System's integrity algorithm, we analyze the XPL value which the system can provide in China Mainland. We obtain the conclusion that the COMPASS can provide the integrity of APV-Ⅰin China Mainland under a single frequency lastly.