Abstract: The earth magnetic field is a small vector.A system including two sensors of Hall-effect is build to measure the horizontal component of the earth magnetic field using the rotating coordinate method.In addition,it deduces theoretical result with matrix calculation which correspondes to experiment.The result of measurement of Qingdao is 3.16×10-5 T and the relative error is 0.3%.

Key words: horizontal component of the earth magnetic field, Hall-effect sensors, rotating coordinate system