Tooth surface crack is an early fault before spalling, which has an important influence on mesh stiffness and vibration characteristics of the gear system. However, the researches on tooth surface crack are limited as scholars pay little attention to this early fault. In this study, an analytical model of spur gears with tooth surface crack is established. Using the potential energy method, the equations for mesh stiffness calculation of spur gears with tooth surface crack are derived. By adopting the proposed model, the influences of tooth surface crack fault on mesh stiffness of gear tooth are studied. The relationship between tooth surface crack and mesh stiffness of gear tooth under different lengths and depths can be further calculated. This study provides a theoretical basis for the diagnosis of early failure of spalling.