Frequency Scanning Interferometry (FSI) absolute distance measurement technology exhibits significant potential for applications in advanced manufacturing and aerospace technology. To address the issue of significant measurement errors in traditional FSI systems under high?dynamic conditions, this paper proposes an FSI distance measurement method based on electro?optical intensity?phase cascade modulation. Based on the theoretical derivation of the amplification effect of optical path difference variation in traditional FSI systems, a double?sideband FSI distance measurement system with electro?optical intensity?phase cascade modulation was designed. The system employs a single photodetector for photoelectric detection of the frequency?scanning interference signal, and uses all?phase fast Fourier transform (APFFT) to extract the phase information of the interference signal with high precision, enabling the simultaneous high?accuracy measurement of both absolute distance and relative displacement of the target. Simulation results show that the standard deviation of the absolute distance measurement of this method can reach below 10 μm, and the standard deviation of the relative displacement measurement can reach below 10 nm, which effectively verifies the feasibility and accuracy of the proposed method.