In order to meet the requirement of high precision when large annular trust plane is gridding, surface sampling is required for flatness. Joint model of measurement based on structured light was propounded. The projector transmitted parallel fringes onto the measured plane, CCD camera caught the images including fringes. When the measurement plane was perfect, CCD camera would catch straight fringes. When there was convex and concave in the palne, the fringes CCD caught would be distorted. There was a set proportional relationship between the distortion and the relative height of the measured point. This paper derived combined model of Structured Light, analyzed the effect of incident angle to the precision. Result of experiment showed: the more incident angle, the higher the precision. As the incident angle increased, the precision would meet the required precision. However, as the incident angle increased, the image was twisted severely and hardly dealt with. A develop model was propounded. CCD camera was located above the measured plane, and array laser scanned the measured plane from a set incident angle. Helped by sub-pix, when the plane with size 400 mm×400 mm was measured, CCD 4 096 pixel×4 096 pixel applied, incident angle 85°, the precision will be the level of submicron. Key words: large annular plane; flatness; surface sampling; line structured light