As a core part of subsea production systems, subsea control modules (SCMs) are costly, difficult, and expensive to install and inconvenient to use in underwater maintenance. Therefore, performance and function tests must be carried out before launching SCMs. This study developed a testing device and an SCM test by investigating SCMs and their underwater. The testing device includes four parts:a hydraulic station, an SCM test stand, a signal generating device, and an electronic test unit. First, the basic indices of the testing device were determined from the performance and working parameters of the SCM. Second, the design scheme of the testing device for the SCM was tentatively proposed, and each testing device was designed. Finally, a practical measurement of the SCM, in combination with the hydraulic station, SCM test stand, signal generator, electronic unit, and high-pressure water tank, was carried out according to the test requirements. The measurement mainly involved equipment inspection before testing and an experimental test for the SCM. The validity and feasibility of the testing device and method were simultaneously verified through an association test.