The invention relates to an improved atmospheric dispersion calculation method for a potential consequence of a design basis accident of a nuclear power plant. The method comprises the following steps of (a) selecting a radioactive substance release mode in the accident, and obtaining hourly meteorological data, a sub-direction distance of a non-residential area boundary, a sub-direction distance of a planned restrictive area outside boundary, and atmospheric diffusion parameters; (b) calculating 0-2h atmospheric dispersion factors of the non-residential area boundary and the planned restrictive area outside boundary, performing special processing for a small quiet breeze condition and hourly meteorological data input, and further giving out conservative values and actual values of the atmospheric dispersion factors; (c) calculating annual average atmospheric dispersion factors of the non-residential area boundary and the planned restrictive area outside boundary; and (d) according to calculation results of the 0-2h atmospheric dispersion factors and the annual average atmospheric dispersion factors, calculating atmospheric dispersion factors of the non-residential area boundary and the planned restrictive area outside boundary in other time periods, and giving out conservative values and actual values.