To improve the current accurate calculation of PMSM based on MTPA control mode, the classic mathematical formulawas analyzed and the cause of calculation error was found, that is, for the interior PMSM, parameter Ld almost does not change with theid value, but Lq varies with iq value greatly, so the partial derivative of Lq to iq should be considered. Based above, an optimaized accuratecalculation formula was provided. Many PMSM were test ,the experimental data was good agreement with calculating data , the accuracyof MTPA current calculation method was validated.