Adopting the grey system theory, the carbonation equations for concrete with different water cement ratios and different fly ash contents are established, different grey models（GM）are compared and analyzed. Based on the forecasting accuracy variation of the models, a basic method to establish grey forecasting model for concrete carbonation depth is therefore proposed. Experiments show that GM forecasting is of good pertinence and high analog accuracy, and is capable of selfupdating when new information is found. As a conclusion, GM forecasting is a feasible and practical method with many field of applications.