The present invention discloses a modulation device for generating an optical signal with quadruple frequency and the modulation method thereof. The modulation device in the present invention utilizes a commercial integrated modulator, a RF signal generator and a phase shifter to generate an optical signal with quadruple frequency. When the RF signal generator generates a first modulation signal, and the phase shifter shifts the first modulation signal by 90 degrees to generate a second modulation signal, the integrated modulator is biased to transmit the optical signal in maximum value and to modulate the first and second modulation signal so as to generate a output optical signal with quadruple frequency.