Abstract: On the basis of the theory of Blewitt,a new method is introduced.The new method uses the ionosphere-free combination LC.Differencing LC data between pairs of satellites is not effective to the ionospheric delay and receiver clock variations.Therefore it is useful for cycle slip detection.The wide-lane combination,the ionospheric combination and satellite difference of the ionosphere-free combination are all used to detected cycle slip.The results show that the combination of the three methods can accurately detect cycle slip.