As the applications of drilling mining technology are extended from petroleum and salt well to trona solution and CBM mining industries, the thin or weak ledges can provide only a smaller target area, which makes the target-hitting more and more difficult. Different kinds of ranging technology depend on different size of target cavities so as to realize the connection between the drilling well and target well. In this paper, the analysis is focused on the deviation errors produced by MWD and rotary magnet ranging system, and the appropriate size of the cavity development is proposed.