In order to quickly and accurately locate the fault of CNC software and improve the reliability of CNC system，an improved algorithm of similar path and fuzzy judgment is introduced into the fault location of CNC software. Firstly，according to the execution information recorded by monitor，the failed path is generated；secondly，the similar path set can be obtained from control flow graph and failed path；then the search scope of suspicious codes can be reduced by program slicing；finally，the fuzzy mathematics is performed to judge suspicious codes. In addition，the experiment on the NURBS interpolation injected bugs is carried out. The experiment results indicate that the proposed method is effective in locating the fault of the CNC software.