As the key components of the subsea Christmas tree system, underwater valves and wellhead connectors are faced with extremely complex marine service environment, and must be tested to verify that they can meet the performance requirements of relevant standards and specifications for opening, closing, locking, unlocking, sealing, pressure bearing, load bearing, etc. In order to fully simulate the working conditions of underwater low temperature and high pressure as well as the rated water depth under combined loading, the key testing technologies of underwater gate valves and hydraulic wellhead connectors based on API Spec 17D and API Spec 6A standards were studied; a set of combined testing methods, procedures and testing systems were proposed; and the laboratory testing application of domestic equipment was successfully completed.