Abstract: It must use the precise P-code for detecting and repairing cycle slip by using classical method with pseudo range and carrier phase combination observations, so its application area is limited. It is improved by considering the combinations for calculate ionosphere effect remains, the improved formulas are derived, the conditions of combinations which can detect and repair cycle slip are also given. Two experiments are made to test the effect by using L1,-1、L-3,4、L-7,9 and L－14，18 combinations. Results show that the improved method can detect and correct all cycle slips when using static observations and almost whole cycle slips when using kinematic observations. Key words: GPS; combination observations; cycle slip; detecting and correcting