The power network planning with new energy, such as small hydropower, generally uses stochastic programming method, which hardly to accurately obtain the probability density function of radial flow hydropower station output. Hence, in this paper, the distributional robust optimization method is used to solve the minimal loadshedding problem of the power network planning contained small hydropower with uncertain output. Furthermore, the minimal loadshedding calculation model based on the distributional robust optimization theory is built, and the uncertain grid planning is turn into a certain semidefinite programming by Lagrangian duality principle. Finally, there are calculating the minimal loadshedding under different hydropower station capacity, and the results show that the sensitivity of different nodes is not same and the minimal loadshedding is influenced by the change ranges of radial flow hydropower station output, which increases with the change ranges and is proportional to the gridconnected hydropower station capacity.