At present, not many studies have considered methods to quantitatively evaluate the reaction efficiency of fly ash at different curing temperatures. For high volume fly ash mortar, when the replacement ratio exceeds a certain ‘threshold’ value, the superfluous and ineffective fly ash will no longer react in mortar but simply behave as a fine aggregate. In this study, experiments on mortars with different replacement by fly ash ratios were conducted at different curing temperatures （20, 30, and 50℃）, and the amount of Ca（OH）2 and strengths were comprehensively analyzed to determine the threshold value of the effective replacement ratio by fly ash. The results showed that the threshold value of effective replacement ratio can be considered as the turning point of the strength curve with replacement ratio. The threshold value of effective replacement ratio by fly ash decreased with increasing curing temperature, whereas the reaction efficiency of fly ash increased with increasing curing temperature. Meanwhile, the analysis of cement effective coefficient （k value） and basicity was also calculated. Based on the obtained threshold values of effective replacement ratio at different curing temperatures, the formula for the determination of reaction efficiency coefficient of fly ash in the mortar can be determined. The reaction efficiency of fly ash can be described more intuitively and quantitatively.