The inclination cannot be easily measured by the near-horizontal freezing hole. As a result, the principle of horizontal fiber optic gyroscope inclinometer and the measuring principle of top angle and azimuth angle are analyzed, comparison is made between freezing vertical hole and horizontal hole in terms of influence of measurement error of azimuth angle on measuring precision; and the method of pre-lowering plastic pipe and hinged connected top is put forward through field measurement. The results show that: (1) The top angle measuring method of gravitational acceleration in horizontal fiber optic gyroscope inclinometer is reliable; compared to lighting inclination method, the maximum measuring error along vertical direction of the borehole trajectory is less than 30 mm. (2) The attitude instability of the horizontal fiber optic gyroscope inclinometer caused by the coupling in the frozen tube can be effectively overcome by using the pre-lowered plastic casing method. (3) Compared to lighting inclination method, the error of horizontal orientation measurement can be controlled within 200 mm. A set of fiber optic gyroscope inclinometer system with horizontal freezing hole and its field implementation method are formed, which solves the inclination measuring problem of horizontal freezing hole and provides a reliable guarantee for municipal and tunnel horizontal freezing construction.