With the increasing number of terrorist attacks, radar imaging systems are not only limited to imaging, but also expanded to the field of detection of some hidden dangerous targets. At present, most of the domestic security screening systems that have been put into commercial application are two-dimensional imaging mode. Compared to three-dimensional imaging, two-dimensional imaging is a lot of limitations, such as spatial blur and shadow effect. Therefore, in order to solve the limitations brought by two-dimensional imaging, near-field radar imaging technology gradually to three-dimensional imaging development, the technology can be applied to a variety of scenarios such as security checks, through-wall detection. In this paper, on the basis of BP algorithm, the formula of 3D BP imaging algorithm is derived, and the effect of different window functions on the paraflap is analyzed. Through simulation experiments, near-field BP 3D imaging is realized to verify the performance of this 3D imaging algorithm, and the effects of different window functions on the paraflaps and resolutions in three dimensions are compared and analyzed.