This paper proposes a method of geometry modeling for indexing cam mechanism by using screw theory and exponential product formula. Kinematic analysis based on screw theory is introduced firstly. Then, method to model surface of indexing cams is presented when screw theory and exponential product formula are applied. The paper gives examples to obtain the geometry models of roller gear cam, barrel indexing cam and parallel indexing cam. Our work suggests that this method can avoid the burdensome work on building coordinate systems, transformation matrixes and understanding on the mechanism. Moreover, this method can be used to innovate and design new types of indexing mechanism.