For the reinforcement of structural member on site, the member can not be unloaded. So this paper studies BFRP constraint preloaded square concrete column under uniaxial load. The stress-strain curve of the specimen is deeply researched into. It is found that the curve is close to the parabola. Based on previous researches, the model of parabola plus linear is determined. Through the processing of test data, the model of peak stress and strain of the parabola vertex is obtained. Moreover, with the theoretical derivation, the complete stress-strain model is obtained. The preload taken into account, the model is closer to actual engineering compared with other models.