The invention provides an eightfold-frequency optical millimeter wave generation method and system based on polarization modulator cascading. The schematic diagram is shown in an accompanying drawing. Through combination of polarization modulators with a polarizer, suppression of an odd-order sideband can be realized. Two polarization modulators are connected in series to realize twice suppression of the odd-order sideband. Meanwhile, only a pi/2 phase difference exists between radio-frequency signals loaded on the two polarization modulators, and the modulation indexes of the polarization modulators are controlled, so that other sidebands other than positive and negative four-order sidebands are suppressed in order to generate an eightfold-frequency optical millimeter wave signal. Through the frequency beat function of a photo-electric detector, an electric domain millimeter wave signal is obtained, and the frequency of the signal is eight times that of a radio frequency local oscillator. According to the method, direct-current offset does not need to be controlled, and no optical filter is needed, so that generated millimeter waves have the advantages of high stability, high frequency spectrum purity, adjustable frequency, wide adjustable range and the like.