The invention belongs to the field of hydraulic test equipment and relates to a large-scale unsteady flow decompression box for a lab. The large-scale unsteady flow decompression box comprises an upstream reservoir box, a downstream reservoir box, a test working section, a computer, an air-collecting tank, a vacuum pump, a reservoir air suction pipe and a test working section air suction pipe, wherein the air-collecting tank is connected with the vacuum pump; the reservoir air suction pipe and the test working section air suction pipe are connected with the air-collecting tank; the reservoir air suction pipe is connected with the upstream reservoir box and the downstream reservoir box; a downstream reservoir air suction pipe gate valve is arranged on a downstream air suction branch pipe; the test working section air suction pipe is connected with the test working section; a test working section air suction pipe gate valve is arranged on the test working section air suction pipe; a downstream reservoir air inlet pipe is arranged on the downstream reservoir box; and a downstream reservoir air inlet valve is arranged on the downstream reservoir air inlet pipe. The large-scale unsteady flow decompression box is easy and convenient to operate; a water pump is not arranged in a water return pipeline, so large-flow unsteady flow can be provided, the technical problems that the water pump is subjected to cavitation damage and the original unsteady flow continuous process cannot be stimulated in the decompression test can be solved; the water pump, for adjusting the upstream and downstream water flow, of the common decompression box is saved; work load and investment of civil engineering are saved; the test flow is simplified; water charge and electric charge are saved; and thetest time is shortened.