Passive millimeter-wave (PMMW) imaging has been widely adopted in earth remote sensing and security screening applications. To obtain more information about the interested objects, polarimetric measurement is an important approach. In this paper, the polarization characteristics of several common structures (i.e., sphere, cone, cylinder and human) have been analyzed theoretically and the corresponding physical meanings have been revealed. The analyzed polarization feature parameters include Stokes vector, angle of polarization, degree of linear polarization, and linear polarization ratio. Additionally, the indoor and outdoor simulations have been conducted to display the influence of ambient radiation on the polarization characteristics. The simulation results can provide some intrinsic mechanisms to acquire objects information, such as 3D structure feature and material composition.