The present invention discloses a detecting and determining method of a fractional harmonic source. A or a plurality of fractional harmonic sources is (are) detected in a system by measuring and calculating the fractional harmonic active powers. When a plurality of fractional harmonic sources exist, the responsibility which should be shared by each fractional harmonic source is quantized. The method comprises the following steps of obtaining the voltage signal of a transformer station bus by utilizing a voltage transformer in a power system; utilizing a current transformer to obtain the current signals of lines which are connected with the bus; transforming the signals into digital data via an AC data acquisition device; calculating the effective values and phases of frequency components; selecting fractional harmonic component having the maximum effective value; calculating the fractional harmonic active powers on the lines respectively, determining the one having the negative fractional harmonic active power as the fractional harmonic source; if a plurality of fractional harmonic sources exist, quantizing the influence degrees caused by the fractional harmonic source respectively according to the proportions of the respective active powers and a total active power.