Image quality is the key performance of the optical lithography tool and it relies on the performance of wafer focusing and leveling measurement system. A measurement model of single probe spot for focusing and leveling measurement system is established. The relationship formula of the height of exposure field and the position on sensitive detector is simplified according to the approximation calculation rules derived from wafer mapping standard and integrated circuit size standard. Height and tilts of the exposure field can be calculated on basis of multiple probe spots by using least square method and surface fitting with a plane. Comparing with traditional model, this model can analyze focusing and leveling measurement system with higher accuracy of 10 nm, and can be used in developing lithographic tool with less than 100 nm critical dimension.