The standardized indices are known to evaluate historical drought and Pluvial conditions in long-term records derived from observations or land surface and hydrological models. However, the standardized index computed by short-term series can cause a large-scale error, limiting these applications on short-time series. We find the probability density function changing with the series length, controlled by three parameters (α, β, γ) in the fitting process, influencing the drought and pluvial classification. Therefore, we separate the fitting of three parameters from the standardized process. The three parameters based on long-term series can be used to evaluate drought conditions in short-term series and equal to the results from long-term series drought evaluation.