The day-ahead economic dispatch model based on the standard AC power flow equation is beneficial to obtaining accurate solutions. However, it is a nonlinear programming model and has deficiencies in model convergence and computational efficiency, which limits its application in actual power systems. In this context, an economic dispatch model based on improved convex hull theory is proposed to achieve the efficient and accurate solution. Firstly, the day-ahead economic dispatch model based on the standard AC power flow equation is introduced. Secondly, the convex hull theory and the convex hull equation of nonlinear functions are introduced, and then a day-ahead economic dispatch model based on the improved convex hull theory is proposed. Finally, the IEEE 14-bus system, the 118-bus system and a provincial actual power system in China are employed for simulation analysis. The simulation results illustrate that the proposed strategy in this paper can significantly improve the computational efficiency of the model and reduce the relaxation error, which has the higher practical application value.