In order to fill in the gaps of the current standardized precipitation index (SPI) obeying a certain distribution, the SPI index was calculated according to the distribution of the maximum entropy theory. Taking the Dongjiang River Basin as a study case, the theory of maximum entropy distribution, Gamma distribution, Weibull distribution and lognormal distribution probability density function were used to fit for different time scales of precipitation data respectively. And AIC, KS, AD were used to test the results of fit goodness. The calculation of SPI with the maximum entropy theory and the Gamma distribution were comparatively analyzed. The results show that: when being compared with other three kinds of distribution, the distribution of the maximum entropy theory probability density function fits better for 3, 6, 12 months of precipitation data from 15 stations in the Dongjiang River Basin; in the case of extreme drought (flood), relative to the Gamma distribution, the SPI calculated with the distribution of the maximum entropy theory is smaller (larger), which suggests that the distribution of the maximum entropy theory is more sensitive to deal with the extreme drought (flood).