Based on the data obtained from static loading test of drilling bottom-enlarged grouting piles, this paper has established a model GM (1,1) for predicting settlement of pile foundation by the use of gray theory, and used the modified residual error model GM (1,1) to modify the primary predictors. The results show that the model GM (1,1) can effectively predict the settlement amount of pile foundation. In the researched example project, the predictors of settlement of pile foundation has sued to modify measured ones, resulting in that the drilling bottom-enlarged grouting piles were of better load bearing behavior than the measured results.