The invention provides a three-frequency cycle slip detection and restoration method based on optimal fixed probability, the maximum fixed probability is treated as a target, preferably, three sets oflinear combinations are optimized, and the probability of three-frequency cycle slip detection and restoration is improved. In the existing method, three conditions of long-wave length, low combinednoise and low ionized layer delay are considered respectively, and due to the fact that the three conditions cannot be met at the same time, the existing method is mainly of optimizing a plurality ofsets of linear combinations for cycle slip detection and restoration in a qualitive way, and the cycle slip fixed probability is not high. According to the method in the invention, the three constraints do not need to be considered independently, the three constraints are converted into fixed probabilities, the maximum fixed probability is treated as the target, and linear combination optimizationis carried out, so that the three-frequency cycle slip detection and restoration probability is the maximum, and great significance is brought to fully exerting the three-frequency observation data advantage and finally improving the positioning precision.