1. An X-ray detecting system, comprising: an X-ray detector comprising: a plurality of photo-sensing pixels, each of the photo-sensing pixels comprising to photodiode that generates an electrical detection signal corresponding to an incident light and a switching device that transmits the detection signal; a gate driver sequentially applying gate pulses to the switching, devices of the photo-sensing pixels via a plurality of gate lines, the switching devices of the photo-sensing pixels being turned on by the gate pulses; and a read-out integrated circuit reading out the detection signal generated from the plurality of photo-sensing pixels; and a signal processing unit reading the detection signal output from the read-out integrated circuit and producing a diode signal level from the detection signal, the signal processing unit storing a decay curve of each of the photo-sensing pixels, the decay curve representing a decay tendency of the diode signal level according to a number of gate scans, the detection signal sequentially including an offset image, an X-ray image following the offset image, and a dark image following the X-ray image, the incident light being applied to the photo-sensing pixels during a time period between the offset image and the X-ray image, the signal processing unit setting the diode signal level at a signal level of the X-ray image if the signal level of the X-ray image is less than a signal saturation level, the signal processing unit setting, the diode signal level at a signal level of the decay curve corresponding to a signal level of the dark image if the signal level of the X-ray image is no less than the signal saturation level, the gate driver sequentially applying gate pulses to the gate lines to perform at least one gate scan during a time period between the X-ray image and the dark image.