Using the compass function of smart phone and the uniform magnetic field generated by Helmholtz coil, the horizontal component measurement experiment of the earth's magnetic field in common physics experiment is reformed.  The advantages of high sensitivity, no eccen- tricity error and accurate reading of the compass of the mobile phone are fully utilized, and the experimental error is reduced. At the same time, it improves the purpose of using common tools in life to solve physics problems and stimulate students' interest in learning.