For providing the basis of fiber optic crack sensing for engineering monitoring, concrete sample tests were performed. Based on the experimental results，the effects of the crack orientation's randomness on the multiplexing capability of distributed fiber optic concrete dam crack detection
were analysed quantitatively.It is found out that as the angle between the optic fiber and crack increases, the optic fibers multiplexing capability enlarges to a greater degree, indicating that the crack orientations randomness has great effects on the multiplexing capability of fiber optic crack sensing. Finally , the mechanism of this effect was analyzed, and the corresponding technical means of improving the multiplexing capability of distributed fiber optic crack sensing were roposed.