Abstract: It is difficult to insure the quality of composites for the reason that there are complicated processes during curing process of composites. The intelligent systems of in time monitoring the cure process need advanced sensors though they are effective ways of elevating the quality of composites. In this paper, a new sort of fiber optic sensor was designed, and the variation of refractive index of resin surrounding the embedded fiber optic was observed by measuring the speckle spatial spectrum at the end face of the fiber optic. The resin cure experiment results measured by the fiber optic sensor were given. This sort of fiber optic sensor was found to be able to detect the gel point, the end point and the lowest point of viscosity of resin matrix. This fiber optic sensor system is sensitive to detect the cure process of composite curing in autoclave and can be used in the intelligent system of in time monitoring the curing process of composites for the reason of precision, simplicity and reliability.