In order to get the horizontal angle of the head of full face tunnel boring machine, a fiber optic gyroscope strap-down north-seeker is proposed, effectively avoiding the problems of bad visibility and smoke interference when using laser guiding system. The horizontal component of the earth's angular velocity is measured by fiber optic gyroscope, and elevation angle and roll angle of the tunnel boring machine is measured by inclinometer. Tilt error of gyroscope sensitive axis is compensated by coordinate rotation transformation. Moreover, latitude is eliminated by structural formulas using the components of the earth's angular velocity in the carrier coordinate. The horizontal angle of the tunnel boring machine can be calculated by four-position algorithm. The strap-down north-seeker is installed in a full face tunnel boring machine and can achieve 0.07° seeking north precision in the experiment of working condition, which satisfies the requirement of tunnel construction.