Embodiments of the present disclosure disclose a signal modulation and demodulation method and system, and an apparatus, to eliminate SSBI generated in a modulation and demodulation method. The method includes: performing, by a transmit end, PAM encoding on an input signal to obtain a PAM signal; electrically modulating, by the transmit end, the PAM signal by using a first filter; converting, by the transmit end, the electrically modulated PAM signal into an analog signal; optically modulating, by the transmit end, the analog signal by using an electro-optic modulator to obtain an optical signal, and sending the optical signal. Another method includes receiving, by a receive end, the optical signal, and converting the optical signal into an electrical signal; electrically demodulating, by the receive end, the electrical signal by using a second filter; and performing PAM decoding, by the receive end, the electrically demodulated electrical signal to obtain a restored input signal.