This invention provides a position monitor method and a system , in which, said method includes: A, designing a mobile admitted boundry of a monitor target and a mobile admitted region corresponding to said boundary, B, plotting out caution regions contained in the mobile admitted region including caution regions in the admitted region sphere and those passed by the boundary or pressing the edge, said caution region is a square and the transverse edge of said square is parallel to the woof, every caution is corresponding to a parameter including the longitudinal and latitude value of an any opposite angle of the square on an electronic map, C, following the position change of the monitor target to report the current position information including longitudinal and latitude value of the current position of the target, D, determining the caution region the target places based on the information of the target to issue beyong the mark alarm information when the target overstepping the mobile admitted region.