The effects of crystallization fraction on the optical constants of Ag 11 In 12 Te 26 Sb 51 phase change films were studied. The phase change thin films were crystallized using initializer and different crystallization fraction was obtained by changing the initialization parameters. With fixed initialization velocity, the refractive index decreases and the extinction coefficient first increases then decreases when the laser power increases. With fixed initialization laser power, the refractive index increases and the extinction coefficient first increases then decreases when the initialization velocity increases. The changes between amorphous phase and crystalline phase, the changes in microstructure (including changes in crystalline phases and the bonding state), and the internal residual stresses in film are the main factors influencing the complex refractive index of Ag11In12Te26Sb51 phase change film. The transmissivity of single Ag11In12Te26Sb51 film and reflection spectra of the Ag11In12Te26Sb51 film in CD-RW phase-change optical disk at amorphous and crystalline states were measured.