Abstract: The fracture phenomenon of Johnson mesh strand in ammonia synthesis tower occurred during operation. The fracture reason was analyzed by means of macroscopic observation, microscopic analysis, mechanical property test and fracture analysis. The results show that the nitriding layer was formed on the surface of the Johnson mesh strand, which led to a significant increase in brittleness. During the formation of the surface nitriding layer, endogenous microcracks appeared inside the strand, and the cracks further expanded, resulting in the fracture of the Johnson mesh.