According to the projection characteristics of the overlapping glass, based on the laser projection method, a new method of measuring the size of the overlapping glass is proposed--- the mobile measurement method based on the laser reflection projection principle. The new method solves the problem of using the non-contact measurement method to measure plane size of the overlapping glass. The method obtains the projection image of the laser projection line after the glass reflection, analyzes the projection characteristics of the laser line, and extracts the characteristics of the glass plane size and the plane size of the overlapping glass is calculated by this feature analysis. According to this method, a set of overlapping glass plane size measurement experiment system is designed. The experimental data show that the measurement system can meet the requirement of production precision and is feasible for the measurement of the plane size of overlapping glass. It can be applied to the on-line measurement of overlapping glass plane size on the pipeline.