The gain calculation of a whispering-gallery-mode fiber laser pumped by an evanescent wave involves in the distribution function of incident angle on the inner surface of the fiber, and the numerical calculation. Based on the ray-optic theory, the distribution function of incident angle on the inner surface of the fiber is derived as the beams of Gaussian distribution and beams of homogeneous distribution are tightly focused via a concave lens and coupled into an optical fiber. Distribution function is caculated by using the method of composite Simpson integral formula, and the gain property of a whispering-gallery-mode fiber laser has been successfully investigated as the pump light propagates along the inner surface of the fiber via frustrated total internal reflection. The calculated results have reference values for the theoretical and experimental studies for this kind of fiber lasers.