This paper establishes a hybrid power network equation with node voltage and branch current as state variables. The characteristic equation representing the stable boundary of the system is derived. Then the boundary conditions of static stability of the power system on the critical circle of voltage static stability are formed. Find the closest distance equation between the load point and the corresponding stable boundary through geometric analysis. The introduction of the distance equation avoids artificially setting the direction of load growth. The distance equation and boundary characteristic equation are added to the hybrid power network equations as additional equations for analyzing the minimum load boundary. The equations are solved by Newton's iterative algorithm. In order to avoid the Jacobian matrix singularity near the load boundary, the algorithm adopts the method of replacing the critical point in the Jacobian matrix to calculate. The simulation results show that the method is correct and effective.