A 3.0 THz detector based on plasma-wave theory proposed by Dyakonov and Shur was designed and fabricated in 65 nm standard CMOS process, the detector consists of a patch antenna, a NMOS field effect transistor, a matching network, and a notch filter. The detector can achieve a room-temperature responsivity （Rv） of 526 V/W and a noise equivalent power （NEP） of 73 pW/Hz^1/2. The terahertz scanning imaging system was built with the detector and stepper motor, and the far-field shape of the terahertz source beam was obtained, the full width at half maximum （FWHM） of the beam is 240 μm; and the image of the polyformaldehyde toothpick and tree leaf were obtained through the scanning imaging system, it shows that CMOS terahertz detectors have potential applications in the imaging field.