The invention discloses a subsea X-mas tree hydraulic valve performance online monitoring and diagnostic system, which comprises a data collecting portion and a data processing portion, wherein the data collecting portion collects hydraulic data of an underwater electronic module and/or a hydraulic supply module and/or a production control system respectively, and transmits the hydraulic data to the data processing portion; and the data processing portion stores and processes the hydraulic data, compares and analyzes the hydraulic data by combining pre-stored standard hydraulic data, and outputs detection results and diagnostic information. The invention further discloses a monitoring and diagnostic method corresponding to the subsea X-mas tree hydraulic valve performance online monitoring and diagnostic system. The subsea X-mas tree hydraulic valve performance online monitoring and diagnostic system and the subsea X-mas tree hydraulic valve performance online monitoring and diagnostic method acquire and analyze data online in real time, substantially increase the working efficiency, are not affected by situations such as harsh weather and sea conditions, and eliminate safety risk of personnel and equipment during construction operation in the prior art; and the subsea X-mas tree hydraulic valve performance online monitoring and diagnostic system is applied to monitoring and diagnosis of various kinds of underwater hydraulic valves, does not request large-scale engineering equipment and a large number of operators in operation, and can save a lot of personnel and equipment cost.