A Doppler shift spectroscopy (DSS) system has been applied to the neutral beam injection system on the Experimental Advanced Superconducting Tokamak. The DSS optical diagnostic system has significant advantages for diagnosing the high power neutral beam, such as real-time, no contact. The major components of the system are lens, optical fiber, grating spectrometer, CCD, software and computer. With a spectrum of the H alpha lines, the DSS system gives the beam energy, the beam divergence, and the beam species fractions. The beam species evolution is measured by DSS. The result indicates that there is no significant different for beam species under the long pulse operation and fully illustrate the good performance of the DSS.