The invention discloses a calculation method for a power system optimal power flow based on an external network operation constraint equivalent value. The calculation method mainly comprises the stepsof (1) establishing an external network optimal power flow model under an alternating current transmission line; (2) establishing an external network optimal power flow model under a direct current transmission line; (3) solving a power feasible region of the alternating current transmission line; (4) solving a power feasible region of the direct current transmission line; (5) establishing a power system optimal power flow model in consideration of the external network operation constraint equivalent value under the alternating current transmission line, and solving an optimal power flow of the power system optimal power flow model in consideration of the external network operation constraint equivalent value under the alternating current transmission line; and (6) establishing a power system optimal power flow model in consideration of the external network operation constraint equivalent value under the direct current transmission line, and solving an optimal power flow of the powersystem optimal power flow model in consideration of the external network operation constraint equivalent value under the direct current transmission line. With the calculation method for the power system optimal power flow based on the external network operation constraint equivalent value, an optimal clearing result of a power system market can be accurately calculated under two power market transaction modes of bulk sale and direct power-purchase of the large user.