Abstract: Chapman-Ferraro model is solved for a three dimentional magnetopause.It is proved theoretically that there is a demarcation line with definite length of about 40° through the cusp region.The surface of the magnetopause is angled along this line,on which every point is an exit for the earth's field line to emerge from the magne-tosphere.The neutral point is just the intersection of the demarcation line by the meridian plane.