The main goal of current load forecasting is to improve the accuracy of the algorithm, and it is impossible to quantify the impact of different forecasting accuracy on the dispatch of a new energy grid. Therefore, this paper studies the impact of a different load forecasting accuracy on the cost of a new energy grid and carbon dioxide emissions. First, the multi-objective function and constraints of the new energy grid including wind-solar-water-thermal power units are established. Second, the calculation using a particle swarm optimization algorithm in the optimal dispatch of the new energy grid is discussed. Third, the load forecasting is carried out using a grey algorithm and the prediction results are given. Finally, the different load forecasting accuracy of a particle beam optimal dispatch is given. The numerical results of new energy grid cost and carbon dioxide emissions are presented and the fitting rules are analyzed. The results show that the load forecasting accuracy and the cost and CO2 emissions of the new energy grid satisfy the quadratic function relationship. This can provide a reference for the planning and scheduling of a new energy grid. This work is supported by National Key Research and Development Program of China (No. 2018YFB090500) and Science and Technology Project of Guangdong Power Grid Company (No. 031300KK52160028).