Cement hydration degree, fly ash reaction degree, portlandite(CH) content and chemically combined water(Wn) content in both pure Portland cement paste and fly ash blended Portland cement paste aged from 1 to 360 days were determined quantitatively. Then, the amounts of CH and Wn generated(or absorbed) by complete hydration of unit mass cement and fly ash were investigated respectively. Finally, the accuracy of chemical reaction model of fly ash in cement paste was discussed. The results show that 1g cement combines 0.2351g Wn and generate 0.2425g CH while 1g fly ash reacts with 0.5089g CH and generates 0.1839g Wn. The theoretical data calculated by existing reaction model and experimental data show significant difference. After reaction model amendment, the calculated data are consistent with the experimental data well, indicating that the amended reaction model truthfully reveals chemical reaction of fly ash in cement paste.