Abstract: The basic construction of BeiDou global satellite navigation system (BDS) has played a key role in promoting the development of global navigation satellite system, which provides an important guarantee for multi-constellation fusion positioning, navigation and timing. Multi-system fusion positioning provides users with more navigation information, and also provides a better guarantee for the realization of navigation performance indicators. At present, in order to ensure integrity, it is necessary to warn users when navigation service is unavailable. Due to satellite failure, the protection level exceeds its alert limit. However, under the condition of multi-frequency and multi-constellation, the number of satellites is sufficient. After a few satellite faults are eliminated quickly, the influence on the overall navigation accuracy is very small and no alarm is needed. A protection level reconstruction method based on robust estimation is proposed. The protection level is reconstructed by using robust information and posterior variance.In order to verify the feasibility of the algorithm, the accuracy and protection limits required by localizer performance with vertical guidance 200ft (LPV-200) approach phase are selected as the standard of this paper, and the global positioning system (GPS)/BDS combination of dynamic and static data are used for calculation and analysis. The GPS adopts L1 pseudo range observation, and the BDS adopts B1 pseudo range observation. Each group of experiments is divided into two parts, fault detection and elimination and the method of adding abnormal error is calculated respectively, the vertical protection level is calculated by the protection level calculation method based on a posteriori variance and the protection level reconstruction method based on robust estimation. The results show that our proposed method can reduce the alarm frequency on the premise of ensuring the positioning accuracy.