The invention provides a valve bank differential protection method performing synthesis judgment based on difference stream waveforms. According to the invention, synthesis judgment is performed based on difference stream waveforms of the a valve bank and whether failures are in a protection range or not is identified according to features of the difference stream waveforms. Then adjustment is made on time delay of valve bank difference based on the synthesis judgment of difference streams and differential motion constant values of the valve bank. By adopting the invention, error action of valve bank differential protection due to discontinuous differential stream influence when an alternating current system meets failures can be prevented. At the same time, failures can be isolated with reliable action of differential protection when troubles inside the sample space occur. The method can be widely applied to a high voltage DC power transmission field.