Abstract: To acquire the electromagnetic interfere(EMI) from a electric thruster in operating, it is necessary to execute the thruster's EMI test. During the test, thrusters must be operated in vacuum. A fiberglass side-chamber has been hereby fabricated to mate to a large vacuum stainless steel chamber. In view of design requirement of the chamber, the performance evaluation method for the chamber is proposed from transparent capacity, operating thermal characteristics and ultimate vacuum. The operating characteristics and the evaluating results of the chamber are introduced. The evaluating results offer reliable appling evidence for the chamber used in EMI test.