OLED-related industries have grown swiftly in recent years. The thin film materials, device structure and film thickness all affect characteristics of OLED devices. Among them, accuracy of thin film thickness need to be monitored with different measurement systems. Apart from the traditional method using quartz oscillation film thickness monitoring system to measure the coating rate and thin film thickness, we have developed an optical film measurement method with high accuracy to monitor changes in the thin film characteristics. This method has several advantages, for example, faster measurement speed, non-contact film surface, non-destructive film structure and so on. In the future, the module can be installed on the vacuum coating equipment for in-situ measurement of the optical properties of the thin film, and to optimize the coating process and thin film quality.