The phase property of optical films is becoming a new research focus in optoelectronics. The retardation caused by reflection on multilayer optical films can be utilized to make phase retarders and modulate the polarization state of optical systems. In this paper, a novel method based on laser feedback is presented to measure the retardance. The laser feedback system can realize fast and stable measurement with high accuracy. Two samples of K9 glass covered with different multilayer optical films are measured at different angles of incidence. The results show that the retardance is sensitive to the incident angle and can provide guidance for the usage of reflective optical films.