Terahertz-wave can penetrate most non-metallic and non-polar substa nces,and it can be used in the field of biological detection and imaging with the characteristics of low photon energy.In this paper,terahertz laser pumped by CO_2laser is used to build a fast point scanning system with transmi ssion detection.The sample is placed on the platform,and terahertz beam is focused on the sample surface by t he lens.The platform movement and real-time signal acquisition are controlled by the computer.Compared with o thers,this system can reduce the scanning time greatly.At last,a computer is used for image reconstruction.The terahertz continuous wave scanning imaging system is optimized by increasing the scanning speed and improv ing the resolution of the image. When scanning size is 1cm×1cm and scanning step is 0.25mm,we can get a scann ing image in 3min.When the output frequency is 4.3THz,the resolution can reach 0.1mm.Clear scanning i mages of knife and leaves are achieved.The effect of diffraction limit is analyzed.Selecting high frequency l ight and setting small steps can achieve high quality scanning image.