The feedback information (field data) of the machines from the actual users forms the main data used to analyse their reliability. Due to the fuzziness of the field data, however, it is difficult to analyse their reliability by the field data. Thus, taking the hydraulic truck crane model QY20A as an example, this paper proposed a method to evaluate the reliability of machines with the field data by the fuzzy sets theory. The effectiveness of the method is illustrated through an example.