The invention discloses a power grid planning method based on probabilistic available transmission capability. The method comprises the following steps: S1, establishing models of uncertain factors faced by power grid planning, wherein the models of the uncertain factors comprise a generator capacity uncertain model, a load increase uncertain model and a line fault uncertainty model; s2, establishing a power grid planning model based on the probabilistic available transmission capability, wherein the power grid planning model comprises an objective function and constraint conditions; and S3, solving the power grid planning model by adopting a greedy random adaptive search algorithm. Based on uncertain factors confronted in power grid planning, a power grid planning method considering uncertain factors and probabilistic available transmission capability is established; through a greedy random self-adaptive search algorithm, the line power flow out-of-limit probability and the probability available transmission capacity meeting constraints and the power grid planning scheme meeting the 'N-1' security constraints under the condition of determining information are obtained, and the transmission capacity and the security reliability of a power grid network are improved.