Abstract: There appeares the multi-harmonic source phenomenon in the rural distribution network because many nonlinear loads are put into operation. The computation of harmonics superposition becomes more complicated when harmonic currents are caused by multi-harmonic sources at random phase angel. To solve this problem, an algorithm to analyze and research the multi-harmonic sources was proposed based on certain assumptions, and the calculation formulae of harmonic superposition were deduced based on Big Number Theorem and Central Limit Theorem of probability statistics. When harmonic sources are enough, the expectation can denote the harmonic current value which can provide rational data for harmonic suppression and avoid a waste of resource. The simulation results show that using the method to compute the values of harmonic current caused by multi-harmonic sources, a satisfactory result could be obtained and the harmonic currents by multi-harmonic sources could be counteracted one another.