Abstract: The third frequency carrier phase virtual observation is produced by numerical transform through the physical relationship of each frequency carrier phase observations.Using long-wave character of multi-frequency combination,a group of long-wave combinations are formed to detect cycle slip of GPS double-frequency raw carrier phase observations in different conditions through the pseudorange/carrier phase combination method and get a group of cycle slip candidate.Then we use Melbourne-Wubbena combination get the difference on each epoch of wide-lane to judge the true cycle slip.The result shows that this method can real-time fix the cycle slip of double-frequency origin carrier phase observations.