A novel inspecting system combining a tunable exposing unit for inspecting micro-cracks of solar wafers is proposed. With the infrared ray transmitting through silicon wafers, the inspecting images will be captured by a CCD camera, and the micro-crack defects from the images will be apparently appeared. So it can easier to precisely distinguish either micro-cracks or grainboundaries from the images by certain algorithms. This inspecting system is especially effective when the thickness of the multi-crystalline silicon wafer is not constant. Furthermore, this system is also useful for controlling quality in on-line process of solar cell production.