The design of a compact chamber incorporating multiple functions for a soft X-ray beamline at Taiwan Photon Source is described. This chamber installed to assist the commissioning of the beamline is composed of three major parts: a diagnostic system, an alignment system and a differential pumping system. The capability of the diagnostic system includes flux measurement, monitoring the photon beam, fluorescence imaging and energy calibration of the plane gratings. An alignment system with a diode laser serves to optimize the optical elements at the endstation. The differential pumping system assures a vacuum protection, with beamline pressure kept below 10−9 Torr while the pressure of experimental chambers increased to 10−6 Torr.