The invention provides a power grid power purchase optimization method considering power production external cost. The power grid power purchase optimization method includes the concrete steps that S1 data are collected, wherein data include the contract electricity quantity, the practical power generation electricity quantity, the contact electricity price, the CO2 emissions, the SO2 emissions, NOx emissions, solid particulate matter emissions and the like of all power generation units; S2 the data are analyzed and processed, the power production external cost is calculated, and the step S2 further includes the following substeps that S2-1, the CO2, SO2, NOx and solid particulate matter emission factors of unit power generation quantity of all units are calculated; S2-2, the emission factors of all kinds of pollutants and the control cost of the unit pollutant emission are comprehensively considered, and the power production external cost is measured and calculated; S3 comprehensive power purchase cost of a power grid company is measured and calculated, and power purchase optimization ranking and decision-making are carried out. The power purchase optimization method can obviously improve the grid connection competitive power of clean electric power such as wind power and photovoltaic power and beneficial for smoothly achieving the purposes of energy conservation and emission reduction in China.