Abstract: At present, the GNSS is widely used in various fields of the national economy and national defense, and has become an important space infrastructure affecting national economy and security. However, due to the vulnerability of GNSS itself and the complexity of the external human-made and natural environmental factors, it is easily threatened by electromagnetic interference, network attacks, physical damage, etc., severely affecting the secure application of GNSS high-precision positioning, navigation, and timing services. This article studies the security threats faced by the space segment, ground segment, and user segment of GNSS respectively, and proposes corresponding defense measures to provide strong support for the secure application of satellite navigation systems.