With the rapid development of optical gyroscope technology and manufacturing processes, traditional mechanical gyroscope has been superseded in most fields, and the north seeker with mechanical gyroscope as its core also faces upgrading. If only to replace the inertial device without changing the orientation principle and the development of the north seeker will face a lot of problems that can’t be solved, such as in the low temperature, sloshing and other environments, the accuracy is greatly reduced, the field operation support conditions are demanding. To address this issue, a high-precision north seeker based on rotationally modulated fiber optic INS (inertial navigation system) was proposed. The principle of this north seeker does not merely rely on the projection of the Earth’s rotation in the fiber optic gyroscope for orientation. Instead, it employs fiber optic INS to establish state and measurement equations during the north-seeking process, utilizing optimal estimation methods to obtain the system’s azimuth information. To minimize north-seeking errors, the internal platform of the fiber optic INS was designed to rotate stably around the zenith axis using a reliable rotation mechanism and high-precision locking devices. This approach effectively modulates the equivalent eastward gyroscope drift, which has the most significant impact on north-seeking accuracy, to zero during the rotation period. Additionally, a high-precision north-seeking algorithm was employed to mitigate error factors encountered during the rotation process, thereby enhancing the overall accuracy of the system. In physical tests, the orientation accuracy of the proposed north-seeker is measured to be 3.7″, 6.1″, and 4.2″ under laboratory conditions, high and low temperature environments, and vehicle idle vibration conditions, respectively. These results demonstrate that the instrument possesses high precision and strong environmental adaptability, making it a viable alternative to foreign high-precision north-seeking devices.