A dual-channel electronic signature system is disclosed, having a signature verification server, a signature requester device, and a hand-held device. The signature requester device calculates a characteristic value related to content of a target document, encodes the characteristic value and a destination message to generate a first graph, and outputs the first graph The hand-held device captures and decodes an image of the first graph to obtain the characteristic value, performs an electronic signature operation on the characteristic value to generate a signature data, encodes the signature data to generate a second graph, and transmits the second graph to a destination network address. If the signature data contained in the second graph passes a verification procedure of the signature verification server, the signature verification server transmits a verification graph corresponding to the second graph to the signature requester device.