In order to ensure the safety and reliability of the monitoring and control functions of the X-mas tree control system, an improved Bayesian-GO method is used to analyze the reliability of the control circuit of the underwater tree system. According to the normal working condition and leakage working condition, a GO chart is established for the operation paths of the control valves of the production circuit and annulus circuit of the system. The block diagram of the control loop of the system is listed and converted into a GO flow chart according to the corresponding GO model. According to certain mapping rule, the GO chart is converted into a corresponding Bayesian network structure. According to the forward and backward reasoning calculation rules of the Bayesian method, the risk probability of each valve of the control circuit under different operation conditions is gotten. The analysis results show that the control function of the X-mas tree system is affected to some extent under the influence of high pressure and high corrosion in the deepwater environment, and the failure rate of the Subsea Control Module (SCM) is high.