Terahertz imaging has wide applications such as biomedical, security check and so on. With the development of modern information technology, portable terahertz imaging devices are demanding. An imaging system based on CMOS terahertz detector was proposed. The system includes a CMOS terahertz detector, an off-chip ADC, an FPGA, a 2-D step motor, four parabolic mirrors and a terahertz source. The terahertz detector integrates an on-chip patch antenna and a source-feeding NMOS transistor as detection element, which was implemented in 180 nm standard CMOS process. The output signal of the detector was a dc voltage signal which depends on the detected terahertz radiation power proportionally. An external ADC converts the output signal of the detector into digital signal which was captured by an FPGA and then transferred to a computer to realize imaging. In order to reduce the system volume, all above elements were mounted on a set of printed circuit boards(PCBs). Clear raster-scanning transmission images can be obtained by the imaging system with continuous terahertz illumination. The image of metals in an envelope was obtained by the imaging system at 860 GHz.