Abstract: Based on characteristics in construction and operation, a probability assessment method for supply systems with multi-lines and multi-transformers in parallel is proposed in this paper. Using the probability analysis method, a probability model for multi-lines parallel systems and multi-transformers parallel systems is studied on the basis of such hypothesis as that the power transformer and transmission line fault is subjective to exponential function distribution, and the reliability, fault rate, loss efficacy rate of supply systems with multi-lines and multi-transformers in parallel is determined. The 500kV substation in Hengli and Dongguan is taken for an study example, and the result shows that it is high in the probability supply systems with multi-lines and multi-transformers in parallel, however it will be decreasing with the operation period being increasing.