Abstract: This paper presents an equivalent load method forestimation of fatigue crack initiation life under spectrum loading. Aweight factor and a coefficient of correction are added to the formula ofroot mean square of variable-amplitude-loading, respectively, to reflectinfluence of different loading range and mean stress uponvariable-amplitude-fatigue life, and then the correspondingconstant-amplitude-loading instead of the variable-amplitude-loading is usedto estimate fatigue crack initiation life under spectrum loading. The methodis easy to use as it only depends on constant-amplitude S-N curve anduniaxial mechanical property of the material and it does not contain anyundetermined parameters. The results of fifteen sets of random fatigue test dataunder three spectrum types for two materials are compared withthree parallel methods. Miner rule, modified Miner rule and Root-Mean-Squaremethod, and the average life-prediction accuracy of the method isimproved, respectively, by 99.1, 24.6 and 50.0 per cent.