Monte Carlo method is the normal method to assess the navigation accuracy of INS. To improve computing speed, a rapid assessment method of INS navigation accuracy was presented based on environment function matrix. The relation between design indexes and navigation accuracy can be built by the environment function matrix, based on which, the navigation accuracy can be assessed rapidly, and the design indexes can also be assessed according to the navigation accuracy requirement. The method is more rapid compared with Monte Carlo method which relays on large number of trajectory simulation, so it can be used to assess the performance of INS before choosing the INS products.