Abstract: It is a contradiction between accuracy and rapidity of availability analysis for constellation of the global navigation satellite system （GNSS）. The requirements for availability analysis are researched firstly, and then the basic definition for availability of the constellation is analyzed, the method for analysis based on failure subsystem is proposed, the algorithm for single satellite based on Markov process is given, and the model of service availability based on constellation state probability is developed, the availability of the constellation can be calculated with the model and the basic principles of navigation. Finally, the regional availability performance of the BeiDou satellite navigation system is analyzed based on the proposed architecture and method. The results show that the proposed methods and models meet the special needs for availability analysis of GNSS.