The invention discloses an underwater light source testing device, which is used for testing performance parameters of an underwater light source. The underwater light source testing device comprises a pond, a first travel system, a second travel system, a lamp source system and a detection system, wherein the first travel system and the second travel system are arranged on the pond, so that motion in left and right directions, front and back directions and upper and lower directions and rotation in a horizontal direction can be achieved; the lamp source system is fixed on the first travel system; the detection system is fixed on the second travel system; and a lamp source emits light and irradiates the detection system to detect the performance parameters of the lamp source. Water in the used pond is stable in characteristic, small in fluctuation and stable in testing environment; the condition that the lamp source system and the detection system can carry out three-degree-of-freedom motion and rotation in the horizontal direction is ensured through the travel systems; the displacement coordinate of each degree of freedom can be accurate to 1mm and the rotation angle can be accurate to 0.1 degree; and underwater performance testing of the lamp source can be achieved through watertight devices of the lamp source system and the detection system.