In order effectively verify the diagnosability of discrete event system after failure, a method of boundary diagnosability verification based on Petri net was designed. Firstly, according to the structural characteristics of Petri net, the concept of visible reachable graph and its construction algorithm are proposed. Secondly, a verifier based on visible reachability graph and its construction algorithm are proposed to analyze whether the system is diagnosable after the occurrence of faults and calculate the boundary values that meet the requirement of system diagnosable. Finally, an example is given to verify the proposed method. The experimental results show that the design method can effectively verify whether the system has diagnosable ability after failure and give diagnosable boundary value. The calculation process is relatively simple, which can be used for reference to the problem of fault diagnosable ability of discrete event system in practical industrial applications.