Carbon fiber (CF) is an essential functional material focused on widely, especially in civil engineering. The mechano-electric character of carbon fiber layer based on the polymer-matrix and its sensing application to the pre-embedded crack detecting were experimentally discussed in this paper. Through the uniaxial tension the CF layer reveals a good performance by resistance changing with response to the loading. By three-points bending test, the crack of the structure would lead to an evident increasing of the resistance change ratio of CF layer. From the experimental results, the ratio of resistance increasing of CF layer laid on crack will be higher by 14.25% in average than that of CF layer with no crack laid. Thus carbon fiber mat composed with polymer will provides a new kind of potential sensor which can detect the defects of the structure.