A prototype of frequency 8-tupling millimeter-wave photonic generator based on improved two-step heterodyne was proposed and analyzed. In the proposal, two step heterodyne technique was employed to eliminate the phase noise of two phase uncorrelated lasers. Millimeter-wave signal with frequency eight time of driving frequency was obtained. We theoretically analyzed the generators principle and the issue of power penalty induced by lasers phase noise, fiber dispersion and optical time delay. To verify the mechanism and the discussion, simulation was carried out to verify the principle.