The ECEF is an important reference system for the conversion of the geographical horizontal system and the astronomical inertial system and the measurement of the earth's motion parameters. The research on the error characteristics of inertial navigation under ECEF is still insufficient, and the astronomical/inertial combination mechanism is still unclear. The ECEF inertial navigation calculation and arrangement, the derivation of the inertial navigation error equation, the analysis of the inertial navigation error characteristics, and the astronomical/inertial integrated navigation principle are derived according to the inertial navigation error law, and the astronomical/inertial integrated navigation algorithm under ECEF is realized. The simulation results show that the astronomical/inertial integrated navigation algorithm based on the ECEF system can effectively suppress the oscillation amplitude of the inertial navigation position error, and provide an effective idea for the navigation information guarantee of sea and air platforms.