To improve the physical layer security of the optical communication system, an encryption-modulation integrated scheme based on the dual-drive Mach-Zehnder modulator (DD-MZM) is proposed. In this scheme, the XOR encryption operation is realized between plaintext and key in the optical domain on the basis of the vector modulation mechanism in the DD-MZM, and the function reuse of encryption and modulation is achieved. The basic working principle of the proposed scheme and the optimization process of improving the extinction ratio of encrypted signals are described, and the prototype of an encryption-modulation integrated transmitter is successfully developed based on this scheme. The experimental results reveal that the modulation and encryption of signals with a transmission speed of 32 Gb/s can be successfully realized by this scheme, and the signal extinction ratio is as high as 13.2 dB. In addition, simulation analysis indicates that compared with the common on-off keying (OOK) system, the proposed system has a better extinction ratio for its received signals.