With the deep development of baseline spectrum or severe spectrum, it is becoming more urgent that how to develop a load spectrum and how to determine the fatigue life scatter factor when the measured data is used to develop the load spectrum. Based on the normal distribution theory, this paper discusses the factors that affect the dispersion of fatigue life, especially the difference between the baseline spectrum and the severe spectrum, and then proposes an idea and a method of the conversion of the baseline spectrum to the severe spectrum. The spectrum conversion factor is calculated and compared by the baseline spectrum and the severe spectrum developed by some measured data. The results show that both spectra can be converted to obtain the value of dispersion coefficient.