Abstract: A 3 axes compass module is calibrated in ideal magnetic environment, and standard calibration parameters are put forward. The output of compass module is measured and compared in ideal magnetic environments of single, two and three alternating magnetic fields. Real-time alternating magnetic density about compass module in two representative states was measured and the result was given. The result shows that the precision of compass module decreases with the increase of the magnetic density, especially at heading direction, which agrees with specific sensitivity theory of compass. The reason of this phenomenon is given and the method of improvement is suggested.