According to the characteristics of small fracture strain,the silica fiber is easy to be broken in real application.Thus,silica fiber is replaced by plastic optical fiber (POF) in this paper.A set of plastic optical fiber sensing device,which is composed of light source,bare fiber of plastic optica l fiber and light dynamometer,is designed.Two types of plastic optical fibers,whose diameters are 0.25mm and 0.5mm, respectively,are applied to the sensing device.Then the feasibility of applying both two types of plastic optica l fibers in crack monitoring is verified.The results show that the light intensity in both two types of plasti c optical fibers decreases gradually with crack propagation when crack width is between 0and 6mm.The trend of chang es is more obvious at the beginning of crack propagation and is weakened gradually whe n the crack width levels off to 6mm,which is consistent with theory results.Moreover,the change of light intensi ty is more obvious when the core diameter is bigger.Therefore,plastic optical fiber can perceive crack pr opagation effectively and the sensitivity of crack monitoring is higher when the core diameter is bigger.