The invention discloses a tail chamber of an exhaust system for an engine test bed. The tail chamber comprises a fixed big tail chamber and a replaceable small tail chamber. The fixed big tail chamber comprises a tail chamber convergent section, a tail chamber equivalently straight section, and a tail chamber expansion section. The replaceable small tail chamber comprises a small tail chamber flange and a small tail chamber cylinder body fixedly connected. The small tail chamber cylinder body and the tail chamber convergent section are coaxially disposed in the tail chamber convergent section, and are connected to a tail chamber convergent section end. The fixed big tail chamber is fixedly connected to a tail chamber support disposed in a high-altitude simulation cabin, one end of an expansion corrugated pipe is connected to the outlet of the tail chamber equivalently straight section, and the other end is connected to the high-altitude simulation cabin; the engine and the tail chamber convergent section are disposed in the high-altitude simulation cabin, the tail chamber expansion section is disposed outside the high-altitude simulation cabin, and the engine is a supersonic engine; and the fixed big tail chamber is provided with a circulating cooling water device and a cooling water spraying device. The tail chamber is advantageous in that, test requirements of different supersonic engines for ground simulation of high-altitude flight conditions are satisfied.