Because of its superior characteristics, inverse-time zero-sequence overcurrent relays have been put into use in more and more areas. Many scholars propose to set its setting value through optimization method. However, the commonly used protection setting optimization algorithms often suffer from complex and time-consuming iterative processes, as well as unstable results, and there is still room for further optimization. Therefore, a quadratic optimization method of inverse-time zero-sequence overcurrent relay settings in extra-high voltage power grid is proposed. Firstly, the inverse-time zero-sequence overcurrent relay setting optimization model is established. When the time setting value T_p is certain, the relationship between action time t and the starting current I_p is analyzed, and then the linear fitting is carried out to transform the complex nonlinear non-convex optimization model that is difficult to optimize into a quadratic objective quadratic constraint model. Then, based on the MATLAB software platform, the problem is modeled according to the modeling tool YALMIP, and the optimization solving tool Gurobi is used to solve it. Finally, the IEEE 39-bus system is taken as an example to verify that the proposed method is effective and superior.