The invention discloses an electromechanical transient simulation-based conventional direct-current power transmission system simulation method. The electromechanical transient simulation-based conventional direct-current power transmission system simulation method comprises the following steps: acquiring alternating-current voltage and acquiring the state of an inverter on the last alternating-current step length; when the state of the last alternating-current step length is a commutation failure state, judging whether the alternating-current voltage meets a first preset condition or not; ifthe alternating-current voltage meets the first preset condition, judging the state of the inverter to be a commutation non-failure state and acquiring a trigger angle; if the alternating-current voltage does not meet the first preset condition, judging the state of the inverter to be a commutation failure state and acquiring a trigger angle; obtaining an extinction angle according to an extinction angle calculation formula and according to the trigger angle; and when the extinction angle does not meet a second preset condition, judging the state of the inverter to be a commutation failure state or not and solving the conventional direct-current power transmission system according to a first quasi steady model. The invention also discloses an electromechanical transient simulation-based conventional direct-current power transmission system simulation system. Through the adoption of the embodiment of the invention, the simulation precision of the conventional direct-current power transmission system in electromechanical transient calculation can be improved effectively.