The threshold gain of a composite external cavity semiconductor laser is analyzed. By introducing equivalent reflectivity, the formula of threshold gain coefficient and threshold current density of this composite external cavity semiconductor laser are established. By adopting numerical simulation, the influence of the first-order diffraction efficiency of blazed grating, beam splitter reflectivity and resonator output end reflectivity on the threshold gain is discussed and analyzed. Finally, this composite external cavity structure is analyzed and further optimization possibility is explored.