The defective insulator can be detected by measuring electric field, however, the electric field calculation accuracy of the insulator strings is low. Charge simulation method (CSM), which is optimized by genetic algorithm (GA), is adopted to calculate high voltage electrostatic field. The optimal method principle and feasibility analysis is given. The objective function and optimization model of electric field distribution around insulator strings are constructed and analyzed. The results of calculation and analysis demonstrate that adopting the potential of checkpoints near insulator strings as objective function and using GA to optimize the charge of simulation charge, better performance can be achieved for improving ability of global optimization, calculating precision, searching speed, simulating precision of CSM and accuracy of diagnosing defective insulator in the insulator strings.