For the scheme of 80 GHz optical millimeter-wave (mm-wave) generation using two cascaded Mach-Zehnder modulators (MZMs), an exact analytical solution for the optical mm-wave affected by phase shift drift and splitting ratio is derived with the method of expanding Bessel series. The results show that for the carrier and the fourth-order sideband, the influence caused by phase shift drift is dominant, while the first-order, the second-order and the third-order sidebands are influenced by both phase shift drift and splitting ratio. It follows that the undesired sideband suppression ratio is at least 35.9 dB when the splitting ratio deviation is 0.001, and the phase shift drift is 1o. The performance of the system is perfect if the accuracy is achieved.