The invention discloses a method for detecting a default phase of a frequency converter. The method comprises the steps of: sampling an electric signal of one of the three phases of the frequency converter; dividing multiple acquired sampling values into three groups according to the sampling sequence, wherein each group at least comprises three sampling values; operating an absolute value of the mean value of the sampling values in each group; and if the absolute value of the second group is judged to be greater than or less than the absolute values of the other two groups and less than a threshold, determining that the phase is a default phase. By obtaining the absolute values of the mean values of the sampling values sampled for three times, the method for detecting the default phase of the frequency converter, disclosed by the invention, has the advantages of well suppressing the conditions of big actual waveform harmonics and a plenty of burrs, improving accuracy of default phase judgment and being good in practical applicability.