A new method is proposed to realize millimeter-wave sub-carrier generation at central station by using a double-uniform fiber Bragg grating filter (UFBG) scheme. Two optical waves with a spectrum of a millimeter-wave sub-carrier distance heterodyne after detector at base station then we can get the wanted millimeter-wave signal. The advantages of this new method of millimeter-wave sub-carrier generation are analyzed by designing a radio over fiber (ROF) downlink system,and the eye diagrams after detection and interference demodulation in 0 km,10 km,30 km,40 km transmission distance are also analyzed. Moreover,performance of this radio over fiber downlink system could be improved by reducing the bandwidth of band-pass filter of millimeter-wave sub-carrier generator. The proposed scheme may be taken as one of the candidates for the next generation high-speed and large-capability radio over fiber system.