The reflective optical sensor based on a confocal principle is made up of GRIN lens and optic fiber.It features miromation and high resolution and can be used for the measurement of mircostructure,microdimension and surface contours.A theoretical equation for calculating the return loss of fiber sensor with beveled end face and GRIN lens is derived with the Gaussian light beams transmission principle and matrix optics principle.The change of return loss with changing the fiber sensor parameter is explained.Some techniques to refrain return loss are presented.Experimental results agree with the theoretical analysis,and return loss is refrained with different methods to different faces.