The formulas calculating the quantity of wear have been established in this paper on the basis of the wear tests of five fly ashes and the determinate mathematical model of ash cutting, These formulas may be used as a foundation to count the safety flow rate of gas. The test results and the main factors of influencing ash cutting, such as, particle velocity, property of ash cutting and particle size are discussed at length. The relationship between coefficient (a) of property of ash cutting and the characteristics of physical chemistry of fly ash are analysed. (a) is considered as an important index determining the capability of ash cutting.