Abstract: Based on multi-frequency data combination theory,the relation between ionosphere scale factor and noise scale factor of combined observation was derived.Lots of possible combinations of Compass codes and carrier phases were analyzed,from which three linear independent optimal combinations were selected as cycle slip test quantities.These quantities keep cycle slips as integers and are helpful to accurately estimate and repair them.Experiment based on real Compass data shows that,by using the selected optimal test quantities,all cycle slips greater than 1 cycle can be detected.And cycle slips on basic frequencies can be accurately estimated and repaired,if data gap was not so long and ionosphere is quiet.