Terahertz focal plane imaging technology has many advantages such as high image resolution, short time period, simple system and small size, and it has wide application prospect in security and industrial inspection. Because the sensitivity of the terahertz focal plane detectors is still low, most of them can only adopt the active imaging mode, and there is no more acceptable testable performance evaluation parameter and model for THz active imaging system, it is difficult to provide the theoretical basis and method for its system design guide. Based on the characteristics of terahertz source and focal plane detector, the minimum resolvable contrast matched filter model of terahertz active imaging system is established by considering the influence of target - background characteristic, atmospheric attenuation and device attenuation. The MRC model is validated by combining the literature examples and the experimental results of the actual terahertz focal plane active imaging system. The results show that the measured value and the calculated value are basically the same, and the error is within the reasonable range, which shows the validity of the model.