The invention discloses a frequency converter safety operation area calculating method. The frequency converter safety operation area calculating method is characterized by deeply researching the internal structure of a frequency converter, performing systematic research and calculation on the rectification and inversion links in the frequency converter and the mechanical features of operation ofan external motor, and comparing the output voltage and current of the rectification link, the output voltage of the inversion link, and the rotor current and rotating speed of a three-phase asynchronous motor, obtained through calculation after performing theoretical voltage sag every time, with the parameters of the frequency converter and the three-phase asynchronous motor which can operate normally, so as to obtain a safety operation area in which the frequency converter can normally work after the whole frequency converter encounters voltage sag. On the premise of not borrowing experimental equipment, the frequency converter safety operation area calculating method takes the three links, rectification, inversion and motor operation as one integral body to consider, so as to calculatethe safety operation area of the frequency converter, and has the advantages of being high in calculating speed, being high in calculation accuracy, and not damaging the frequency converter and the motor.