The invention discloses a test device for simulating an underground closed space of a coal mine. The test device comprises a test box with an external water injection port and a rock mass arranged inthe test box; a goaf filled with a water-soluble supporting material is arranged in the rock mass, a water injection mechanism is arranged in the goaf, and a first water injection port connected withthe water injection mechanism is formed in the rock mass close to the goaf; after the first water injection port is connected with the external water injection port, water is conveyed into the goaf through the water injection mechanism, and the supporting material is discharged out of the goaf through the external water injection port after being soluble in the water. The test device disclosed bythe invention has the benefit that by taking the water-soluble supporting material as a temporary support for the goaf and injecting the water into the goaf to dissolve the supporting material, the excavation process of an underground reservoir is simulated.