Based on the characteristics of terahertz source and focal plane detector, the minimum resolvable contrast(MRC) 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, with the error in the reasonable range, which shows the validity of the model.