Synthetic Aperture Radar (SAR) Imaging and Time of Flight (ToF) Imaging are two popular approaches for terahertz (THz) 3D imaging nowadays. Both systems above require broadband operations and coherent detection, which requires complicated and costly hardware. Generally, 3D imaging is difficult for those objects with large-curvature features in the case of both optical and THz systems. In this paper, we propose a simple but effective 3D imaging approach, which works in the point-cloud imaging mode. Our in-house THz lenses can focus the THz wave very well and it can gather reflected power within a large angle, thus making the decent 3D imaging feasible. By introducing the entropy concept for threshold optimization, decent figures can be obtained in case of poor raw data with a low signal-to-noise (SNR) ratio. In experiments, our system outperforms the commercial Terahertz Time-Domain Spectroscopy (THz-TDS) system. The proposed method could benefit industrial production, heritage studies, etc.