Based on Miner’s law and probability theory, the probability density function of damage sum under the 2-stage step loading spectrum is proposed, by taking the logarithm normal distribution of the fatigue life into account. The method to determine the scatter of the accumulative fatigue damage is further proposed. A series of experiments under multiaxial 2-stage step loading spectra are conducted on LY12CZ aluminum alloy, which is commonly used in aviation industry. Combined with the test results of the paper and the references, the applicability of Miner’s law is investigated under multiaxial 2-stage step loading spectra, after considering the material dispersion. The result shows that, for the constant path variable amplitude spectra and variable path constant amplitude spectra, since the amplitudes of the two stages are similiar, the applicability of Miner’s law is affected by the variation of the non-proportionality of the multiaxial loadings. Larger non-proportionality leads to better applicability. For the variable path variable amplitude spectra, since the amplitudes of the two stages are significantly different, Miner’s law is not applicable.