A new passive synthetic aperture imaging detection method is presented. The radiation signal of field of view is received and amplified, and then the amplitude and phase information of the amplified signal is modulated to an optical carrier by electro-optic modulator. The optical signal is transmitted by fibers and a fiber array is shaped by the end of the fibers .The image is formed synchronously and directly by an optical system using the fiber array. The imaging approach can be used in extremely high resolution detection at microwave, millimeter wave, and terahertz frequencies. The application of the electro-optic modulation technique in passived synthetic aperture imaging is analyzed deeply and the model of electro-optic modulation is established. And the approximation of small signal modulation in the electro-optic modulation is discussed. By simulating computation and analysis, the intensity constraint of the modulated signal in an electro-optic modulator is gained. Result shows that the full-width at half maximum (FWHM) and signal-to-noise ratio (SNR) with upconversion electro-optic modulation and optical information processing are both better than those with traditional downconversion imaging technique.