Abstract:Due to the objective restrictions and impact

the reliability test data for heavy engineering machinery and equipment are in small samples and poor information. Therefore

there are differences between the results of reliability analysis and actual ones. To make the differences less

we put forward the possibilistic reliability method based on unascertained theory. The method were consisted of three phages which the critical elements selected by the optimality criteria method

limit state equations of main failure modes made by using incremental load method and the mathematical value of load distribution characteristics acquired by unascertained theory. The possibility of a bridge crane was calculated by the possibilistic reliability method. The results showed that the reliability of the structure can be increased by the method.