The characteristics of THz radiation and a THz Focal Plane Array(FPA) detector were researched, and the energy transmitting processing of a continuous wave imaging system in the FPA detector was analyzed. In consideration of the atmospheric attenuation, device limitation, and the signal transfer relation of continuous laser irradiation, target scene and the FPA detector, two parameter models for the imaging area and contrast ratio of the continuous wave imaging system were derived. Then, a continuous wave imaging system was designed. According to derived models, the imaging areas and contrasts of two different objects(one is the envelope with cyclotrimethylene trinitramine (RDX) powders and the other is the plastic case with metal blades) were calculated. The obtained results show that the imaging areas of the two examples are 4.74 cm×6.32 cm , 3.534 cm×4.712 cm, and the contrasts are 0.242 2 and 0.306 respectively. The results are compared with that of the imaing system with the imaging area of 3 cm×3 cm or 4 cm×4 cm from the Massachusetts Institute of Technology(MIT). They are derived in the same order of magnitude, which verifies that the proposed models and method are reasonable and effective.