A terahertz wave imaging system based on the tunable terahertz source with the quasi-Gaussian beam output is reported. In this imaging system, terahertz wave beam is easy to be focused and transmitted owing its spatial distribution with the excellent beam quality factor of M2x＝1.15 and M2z＝1.25. The experimental results show the spatial resolving power of this imaging system is close to the typical terahertz wavelength of 250μm. In the output range of 1.0—2.5THz, the signal to noise ratio (SNR) is higher than 1000dB, and the SNR is above 6000dB at 1.8THz. Based on this system, the terahertz imaging experiments for the several samples were performed, and clear pictures were obtained.