Abstract: One method and detailed operating process including pressure crushing, acid dissolution, particle sizing and high temperature treatment were developed to determine mixing proportion of hardened concrete with fly ash. By using this method, the initial mix proportion was measured for hardened concrete containing 0~40% fly ash with 28 day curing age. The results show that the deviation of the measured value and theoretical value for coarse and fine aggregate content is below 5%. And the difference between the measured value and theoretical value for cement and fly ash content is about 10%. Therefore, this method can be used to deduce the original mix proportion for hardened concrete with fly ash in practical engineering.