The invention discloses a method for calculating the long-age compressive strength of underground-engineering concrete mixed with fly ash. The method includes the steps of firstly, analyzing and calculating to determine the design-age compressive strength of the concrete mixed with fly ash; secondly, calculating the 28-day compressive strength of the concrete mixed with fly ash; thirdly, calculating the long-age compressive strength of the concrete mixed with fly ash. The method has the advantages that the method is simple in calculation formula, the statistical analysis of more than 250 groups of compressive strength experiment results of the concrete, mixed with fly ash, of a large amount of actual underground engineering can reasonably reflect the relation between the compressive strength of underground-engineering concrete mixed with fly ash and the age, the relation between typical-age compressive strength and the influence of the fly ash mixing amount, the full-age compressive strength of the underground-engineering concrete mixed with fly ash can be calculated fast and precisely, and the method is completely applicable to actual engineering design calculation, safety evaluation, service life researches, fly ash mixing amount influence researches and the like and especially applicable to preliminary design and field real-time calculation and analysis.