In order to solve the problem that abnormity of constant output of satellite star sensor in orbit is difficult to be found quickly and accurately, a method of constant output feature detection based on real-time telemetry data of satellite star sensor in orbit is proposed. According to the principle that the quaternion output of the star sensor changes with the motion of the satellite, the incremental change model of the star sensor quaternion is established, and the model is modified with the measured data in orbit. Then, according to the constant output data characteristics of star sensor, the constant output model of star sensor is established and further applied for anomaly detection. Taking an orbiting satellite as an example, the quaternion constant output anomaly of the star sensor is rapidly detected within 1 min after it occurs, which proves that the proposed method is effective and accurate, and plays an important role in ensuring the satellite attitude orientation and energy security.