A modulation device and method, and a demodulation device and method are provided. The modulation device modulates plural baseband signals to generate an optical signal, and comprises: plural modulation units for modulating plural electrical carriers with different frequencies by using the baseband signals respectively, to generate corresponding electrical modulated signals; a synthesizer for synthesizing the electrical modulated signals to generate a single electrical synthesized signal; an optical modulation unit for modulating a single optical carrier by using the electrical synthesized signal to generate the optical signal, wherein signal components corresponding to the baseband signals in the optical signal are distributed on both sides of a carrier frequency of the optical carrier at a predetermined frequency interval. Thus, processing on a high-speed baseband signal may be divided into processing on multiple relatively low-speed baseband signals, overcoming a bottleneck caused by sampling rates of a digital-analog converter and an analog-digital converter.