The landing gear buffer strut was disassembled for repair, it was found that the nut thread of the plunger assembly was occluded and deformed, which was difficult to disassemble. In order to analyze the cause of the fault, the fault analysis and improvement research were carried out. According to the structural characteristics and usage of the plunger assembly, the fault tree of the nut thread occlusion deformation was constructed. Based on the historical test data, residual strength analysis and products status, the bottom events of the fault three which was analyzed and verified. It was concluded that the design defects of the nut and the nut was not assembled in place could lead to the thread occlusion deformation. In addition, based on residual strength analysis of nut thread, when the thread pitch was 1.5mm, the residual strength was 0.76, while the thread pitch changed to p=2mm, the residual strength was 1.4. Based on the problem that the nut was difficult to be disassembled , the method of bidirectional fixing of the plunger assembly and design of the double sleeve type special tooling was adopted to effectively solve the problem. The research results were great significance to the design improvement and disassemble,assembly quality of nut.