Structures often enduring random amplitude load or multilevel amplitude load. However, the test researches on fatigue often using equivalent amplitude load. It is very important for fatigue research that how to accurately evaluate structure’s fatigue properties of random amplitude load or multilevel amplitude load when only using equivalent amplitude load fatigue test results. According to damaging degree equivalent principle of multilevel amplitude load or random amplitude load, deduced equivalent stress amplitude calculation formula using linearity Miner accumulative damage rule and Corten-Dolan accumulative damage rule. Introduced the formula’s using method on fatigue detail or fatigue life evaluation. Utilizing the S-N curve and S-N equation from RC beams fatigue experiment, verified the formula aiming at 3 RC beams’ random amplitude fatigue experiment results, and it is indicated that the formula’s results had quite high precision and approached the experimentation result though it is partial security. The formula operated convenience and it had higher precision compared to similar formula, so the formula suit for engineering application.