A new algorithm for detecting random harmonic current in unbalanced three phase system is proposed. An arbitrary initial phase angle is constructed, whose sine and cosine signals have the same frequency with the random harmonic current needed to be detected. It is used to conduct correlation operation with three phase current, and the components of positive, negative and zero sequences of random harmonic current can be detected. By adding the three components, the random harmonic current of three-phase four-wire system can be obtained. Without phase lock loop, the circuit is simpler. Both the theoretical analysis and simulation results demonstrate the correctness of the method.