The invention discloses a method for detecting surface leakage channel of semiconductor detection device under illumination. The method judges the existence of surface leakage, that is, surface reflective layer, caused by illumination by measuring capactive differential signal on surface of device in different illumination intensities. By combining the measuring data, the size of the specific leakage channel of the measuring region is obtained through corresponding numerical value simulation. By using this method, the specific surface leakage characteristic of the device is directly and definitely obtained so as to provide parameters having direction for the suppression of the device leakage. The invention has significance on improving the device performance and optimizing the device design.