A technique for passive millimeter-wave scanning imaging based on optical upconversion is designed for millimeter-wave photon imaging radiometer system. The millimeter-wave signal is transferred to the sidebands of the optical carrier by an electro-optical (EO) modulator,and filters are used to suppress the optical carrier. The optical signal of the remaining sideband is converted into a photocurrent by photodetector,and then the photocurrent is converted to the voltage signal by transimpedance amplifier. The lock-in amplifier is used to read the output voltage signal. The brightness temperature image of the objects is obtained by signal processing. We analysis the relationship between antenna scanning speed and antenna temperature contrast,system output signal,noise source,signal to noise ratio,and sensitivity etc. The sensitivity of imaging systems is discussed in detail. Several methods are proposed to improve the sensitivity of the imaging system,which provides a theoretical guidance for the realization of the millimeter-wave radiometer real-time imaging system project.