Effect of voltage hasn't been considered reasonably in existing loss allocation methods. Aiming at the problem, the impact of voltage on the loss allocation method based on circuit theory is deeply discussed in this paper. Firstly, the two node equivalence modes and principles of the branch loss allocation method are investigated. Then the relationship between voltage and the loss under different node equivalence modes is analyzed on the premise that all nodes' injection currents are fixed, so as to keep the influence of current on loss allocation constantly at different voltage levels. The analysis and simulations reveal that none of the methods can reflect the effect of voltage on loss allocation, and this is due to node equivalence rather than branch allocation principle. So a more reasonable node equivalence should be developed to reflect the influence of voltage on loss allocation.