The embodiment of the invention provides a power system operation state determination method and device. Firstly the numbers of a generator node, a parallel capacitor node and a balance node are statically optimized so as to generate a first admittance matrix, and semi-dynamic optimization is performed so as to generate a second admittance matrix; then, the number of non-zero elements generated inthe first admittance matrix and the second admittance matrix is judged and the admittance matrix with less number of non-zero elements is selected to perform power flow calculation on the power system; and finally the operation state of the power system is determined according to the power flow calculation result. Therefore, the power flow calculation efficiency can be improved and the operationstate of the power system with fewer nodes can be determined quickly by comparing the number of the non-zero elements generated in the statically and semi-dynamically optimized admittance matrices andselecting the admittance matrix with less number of non-zero elements.