The invention relates to an electric quantity distribution method and system based on a power grid investment scale, wherein the method comprises the steps of: determining operation target parameters according to power grid enterprise electric power analysis data; obtaining the control variable of a power grid investment scale according to the operation target parameters; performing trend analysis on internal operational data to obtain boundary conditions of the power grid investment scale; obtaining a power grid investment scale analysis value according to the control variable and boundary conditions; and according to the power grid investment scale analysis value, establishing an electric quantity distribution investment scale probability weight function, and obtaining the optimal distribution electric quantity of a power grid according to the investment scale probability weight function. The electric quantity distribution method and system solve an accurate power grid investment scale analysis value according to external energy supply and demand parameters and power grid enterprise internal operational data, establish an electric quantity distribution investment scale probability weight function, and improve power grid electric quantity distribution accuracy.