The invention relates to a method and a system for forecasting the investment demand of a grid. The forecasting method comprises the following steps: 1) selecting variable sequences that have the co-integration relationship and significance and influence the grid investment, and establishing a grid investment demand long-term equilibrium model in advance; 2) solving the grid investment demand long-term equilibrium model to obtain a long-term equilibrium relationship between the grid investment sequence and the corresponding variable sequence; 3) performing the Granger causality test to obtaina causal relationship between the grid investment sequence and the corresponding variable sequence; 4 ) performing the vector error correction test to obtain the corresponding grid investment change amount; and 5) performing logarithm inverse transformation on the grid investment change amount corresponding to each variable sequence, and combining the grid power investment base value of the previous year to obtain the corresponding current-year grid investment short-term adjustment value of each variable sequence. The method and the system can be widely used in the power system grid investmentquantitative evaluation technology.