A light-detection method for a light-detection device including a plurality of scan lines, a plurality of read-out lines and a plurality of photo sensing elements is provided. Each of the plurality of photo sensing elements is coupled to one of the plurality of scan lines and one of the plurality of read-out lines. The method includes simultaneously turning on at least two of the plurality of scan lines to turn on a portion of the plurality of photo sensing elements which are coupled to the turned-on scan lines, turning on at least one of the plurality of read-out lines to transmit signals of the portion of the plurality of photo sensing elements, and determining whether the signals match a trigger standard. When it is determined that the signals match the trigger standard, a reading mode is entered.