Abstract: It is difficult to find out fault chemical oxygen iodine laser (COIL) because of its complex structure and interactions between the subsystems. To solve the COIL fault diagnostic problems, along with the development of integrate and practical COILs, it is required for keeping the device performance and reducing the maintenance cost and human resource input. Fault analysis of COILs has been done based on the fault tree analysis method. The fault tree of a COIL was established combining with on-line monitoring parameters: iodine cavity pressure, iodine venturi pressure, p-t curve, main helium pressure, chlorine pressure, single oxygen generator pressure, upstream pressure, optical cavity pressure. According to the 3 rule, the accuracy of fault diagnosis for the COIL is 94.5%. Furthermore, improvement put suggestions has been forward.