Conventional geometric optics method is a kind of important measurement methods.Combined with optoelectronical techniques

it has advantages of high precision

quick response

easiness to real-time control and noncontact.But its function is liable to be ruined by the defocusensess of both the illuminating and imaging systems

which limit the measurement accurracy of the system and even make the system disable when sometimes the range extends.It is presented in this paper a method to transform the beam by an axicon.It makes parallel beam(or Gause beam)have very good properties of field and energe arrangement in a relatively large depth which improves the system's measurement range without loss of precision.Two simple examples are given to demonstrate its application.