A wide-band spectrum optical film thickness monitoring system with compound light path is introduced. The structure,components and working principles of the system are analyzed. The monitoring system is developed by increasing a piece of spectroscope with aperture to form compound light path,and wide-band spectrum monitoring software based on LabVIEW. Methods of wide-band spectrum optical film thickness monitoring based spectrum scanning method and optical film thickness monitoring based on extremum method are compatible. The accuracy and automatization of optical film thickness monitoring can be improved also. This monitoring system can be used in upgrade or reconstruction of optical coating devices.