In order to monitor the volatilization and deposition of organic contaminants on optics surfaces in high power laser facility, online detection of airborne molecular contaminant was investigated based on quartz crystal microbalance and optical microfiber. First, the response performance of quartz crystal microbalance for different densities of airborne molecular contamination was tested. Then the comparison of quartz crystal microbalance and optical microfiber in detecting the surface mass density of airborne molecular contamination was implemented with the view factor method. The characteristics of airborne molecular contamination deposited on optical surface were also studied. The experiment results show that quartz crystal microbalance and optical microfiber have the similar response performance, and thus they can be applied in online airborne molecular contamination detection. When the surface mass density of airborne molecular contamination is larger than 0.5×10－5g/cm2, optical microfiber is preferred due to the higher precision and larger response speed. The more the density of airborne molecular contamination is, the more it deposits on the optics.