In this paper,we discussed the basic requirement for measurement system to avoid shadows and th frequency alias in a crossed optical axes system in FTP.The maximum range of measurement can be extended by employing a geometry with a smaller included angle between axis of the projection and the detection system;In order to eliminate the errors resulting from the smaller included angle,we project a dense grating image onto a object and record the deformed fringe with a high resolution CCD to prevent frequency alias.Computer simulations have proved our analysis.