A novel optical frequency-hopping (OFH) scheme using dual-drive Mach-Zehnder modulator (DD-MZM) is proposed and demonstrated for secure transmission in fiber-optic networks. In the proposed scheme, by adjusting the phase difference between the radio frequency (RF) signals loaded on the two arms of the DD-MZM, and filtering the center carrier by fiber Bragg grating (FBG), thus the upper and lower output spectra can be realized randomly hop between the positive and negative first-order sidebands and serve as carriers for data modulation. And the data of different users are divided into many data slices in the time domain and then modulated to these carriers. To verify the feasibility of the proposed OFH scheme, we demonstrate an error-free transmission through 40 km fiber with 10 Gbps hopping rate and 10 Gbps data rate by simulation tools. In addition, we also establish a theoretical model to evaluate the security performance quantitatively, and the results show that the computing power required by illegal third parties to crack through brute force reaches 1.0710e27 calculations per second under certain conditions, which is almost impossible. Our OFH scheme has provided a deeper insight into physical layer security.