The scheme of three-self fiber optic gyroscope (FOG) inertial navigation system (INS) has been explored as a potential alternative to provide the desired self-calibration property, and FOG is the main component of the corresponding INS. However, the existence of FOG dead band will cause a severe INS accuracy degradation in process of gyro bias calibration and alignment accuracy. In this article, a correspondence was derived, that is, the ratio of the intersection angle between the FOG input axis and the east direction to the theoretical output of FOG is a constant value in various approximately eastward. Nevertheless, when the intersection angle changes, the ratio also would significantly change because of FOG working in the dead band. Thus, a dead band self-test method at the system level was proposed and the corresponding verification experiments were carried out. The experimental results indicate that the dead band of the $y$ -axis FOG is accurately tested as −0.0125°/h by the proposed method. In conclusion, our work opens up a new perspective regarding the self-test method of the dead band at the system level, and the innovative self-test method is effective and feasible.