Supersonic molecular beam injection (SMBI) is an effective fueling method for the magnetic fusion plasmas. The fueling method was first proposed in the HL-1 tokamak, and now has been applied to several tokamaks and stellarators. Pulsed molecular beam passes from a Laval type nozzle and has a high instantaneous intensity, high directionality and deep deposition in the plasma. The fueling efficiency is higher than the gas puffing efficiency. In addition, it is widely used for controlling plasma density and investigating plasma physics. To further improve the fueling capability in future fusion devices, it is highly desirable to optimize the characteristic of the SMB and further investigate the interactions between the molecular beam and the plasma. In this paper, a schlieren diagnostic system is developed to measure the parameters of molecular beam, and the testing application is performed. The schlieren system, which is based on the schlieren photography, is designed with the zigzag optical path and equipped on the SMBI testing platform to measure the characteristics of the supersonic molecular beam. In order to verify the effectiveness of the system, a series of tests is carried out with different nozzle shapes under atmospheric and vacuum conditions. The beam profiles of CO ₂and D ₂under different background pressures are obtained. The testing results indicate that the directionality of the integrated Laval nozzle is much better than that of the pinhole nozzle. The schlieren system provides a testing tool for optimizing the supersonic molecular beam.