The invention discloses a phase modulation optical signal physical encryption method and system based on optical phase encryption. The method comprises the following steps: at a transmission end, loading a phase encryption electric signal changing along with time rapidly on a phase modulation optical signal of which transmission information is modulated in order to perform phase encryption; and at a receiving end, performing phase decryption on the encrypted phase modulation optical signal by using a phase decryption electric signal which is the same as phase encryption information in rate, equal to the phase encryption information in amplitude, and reverse to the phase encryption information in polarity, and performing demodulation. According to the method and the system, the phase encryption electric signal changing along with the time rapidly is loaded on the phase modulation optical signal to perform the phase encryption, and the phase decryption is performed the encrypted phase modulation optical signal by the phase decryption electric signal which is the same as the phase encryption information in rate, equal to the phase encryption information in amplitude, and reverse to the phase encryption information in polarity, so that physical encryption of the phase modulation optical signal is realized; the cost is lowered greatly compared with quantum optical communication and chaotic optical communication schemes, and meanwhile the system reliability is relatively high.