Most of the existing through-the-wall imaging (TWI) methods using synthetic aperture radar (SAR) tend to apply an active system. In this paper, a novel, passive SAR (PSAR), termed TWI-PSAR, is proposed, to focus the image of multi targets behind a single-/multilayered wall. TWI-PSAR would work in a bistatic configuration using wideband sources of opportunity and a single moving platform or a stationary linear array receiver. Incident angle and frequency are considered the parameters that influence TWI image directly. A stepped frequency transmitter with single incident angle is applied to investigate the incident angle effect. It could show the capability of small angle to suppress wall effects. Zero incident angle PSAR (Z-PSAR) is exploited in TWI for enhanced target identification and feature extraction as well as wall effect mitigation. In scenarios where background measurement might not be available or wall parameters are unknown for compensation, Z-PSAR could be adopted. Compared to other conventional imaging methods such as SAR and time reversal, numerical results show the superiority of the proposed TWI system in urgent situations with unknown wall parameters, employing free-space Green’s function. Moreover, to demonstrate the effectiveness of the proposed PSAR method in a real situation, sources of opportunity that are relatively wideband and aligned in several directions, such as analog TV, Digital Video Broadcasting—Terrestrial, GSM, and WiMAX, are used to image targets behind the wall. Also, Monte Carlo method is used to show the effectiveness of TWI-PSAR in different frequency and incident angle scenarios.