The relationship between the change of a local gravity field and the mass migration underground is discussed by means of using a point-source disturbed body as a substitute for the mass migration underground. Some significant local gravity field changes in Beijing area have been found, from which the three parameters of the disturbed bodies, location, depth and mass, have been derived successfully from the observations of a gravimetric network. In order to determine the depth of a body, a new approach is suggested in which the gravity change difference is used instead of the gravity change itself. The mass of a disturbing body has been estimated properly. Astronomical PZT is suggested for this kind of survey. The results provide us a picture of the underground mass migration with which the gravity changes on the ground surface may be interpreted in a better way. This additional information may be useful in seismological studies.