Abstract: A versatile molecular beam apparatus constructed by Laser Chemistry Laboratory at Fudan University is presented. It consisted of a supersonic molecular beam source with a 3-stage differential pumping system, a reaction chamber with a rotatable sample holder at the center, a quadrupole mass spectrometer situated in the detection chamber and a computerized data acquisition system. A fixed beam source arrangement with a rotatable detection chamber is used for measuring the angular distributions of reaction products. The novel feature of this apparatus is that the detection chamber is connected with two fixed ion pumps for a two-stage differential pumping through two flexible stainless steel bellows. Thus the detection chamber can be rotated around the reaction center. This compact apparatus can be employed to measure the mass selected time-of-flight spectra and determine the mass, velocity and angular distributions of reaction products. This versatile molecular beam machine can be used to do experiments on crossed molecular beams, molecular beam photofragmentation and molecular beam relaxation spectroscopy.