Paper size detection device having a same sensor triggerable by two opposite switches and detection method thereof

A paper size detection device includes a central switch, at least two switches disposed to two opposite sides of the central switch at intervals, and at least a sensor corresponding to the two switches located at the two opposite sides of the central switch. The sensor can be designated as a light sensor. One switch triggers the sensor and the other switch is without triggering the sensor under an original no paper condition. So that a detection method of the paper size detection device that uses N light sensors to judge N*2 papers' size can be realized by virtue of the above-mentioned setting structure, N is a positive integer, such as 2, 3, 4 and so on. Thus, the paper size detection device has a simple structure and a lower manufacture cost, and the detection method of the paper size detection device is simplified, accordingly.