A tunable optical millimeter-wave generation scheme was proposed theoretically which was capable of obtaining high multiplication factor(MF), and the performance of Radio over Fiber (RoF) downlink was numerically analyzed. The scheme was based on a dual-driving Mach-Zehnder modulator(DD-MZM) which was driven by triangular wave instead of sine-wave. More harmonics can be generated by using triangular wave sweep. With the help of uniform fiber Bragg grating(UFBG), two of the harmonics can be selected to generate millimeter-wave signal of different MFs. The MF can be tunable by adjusting the Bragg wavelengths of two UFBGs respectively. Millimeter-wave signals with MFs of 4, 6, 8, 10 and 12 were achieved. The RoF downlink transmission performance of all these MFs was evaluated. The eye diagram after 10 km transmission is still wide open under the conditions of all the MFs. The results demonstrate that such a scheme can offer realistic solution to support future wireless systems.