A light sensing apparatus includes a sheet passage, a first sensing assembly and a first background sheet. The sheet passage has a scan window. The first sensing assembly disposed on a first side of the scan window outputs first scan light to the scan window. The first background sheet disposed on a second side of the scan window has a first reflective surface reflecting the first scan light, passing through the scan window, back to the first sensing assembly through the scan window. The first reflective surface of the first background sheet and the scan window are disposed in a non-parallel manner. Therefore, a light sensing method is also provided to achieve the effects of brightness calibration and boundary detection of the first sensing assembly.