Zero-sequence mutual inductance is an important factor affecting the fault characteristics of transmission lines on the same tower. In order to get its accurate value in a convenient way, the zero-sequence parameters of the transmission lines on the same tower when soil resistance, tower structure, height, and sag changes are studied. The results show that the effect of tower structure on mutual zero-sequence inductance is significant, besides the accepted soil resistivity. For the multi-circuit transmission lines, those circuits which near the earth, whose zero-sequence self-inductances are significantly larger than those which near the overhead ground wire. Based on that, the law of zero-sequence mutual-inductance/self-inductance ratio influenced by soil resistivity is also studied, and a method of zero-sequence mutual-inductance estimation based on voltage level and tower structure is proposed. PSCAD simulation verifies the good robustness of using subdivision ratio to estimate the zero-sequence mutual-inductance of transmission lines on the same tower, and the method’s accuracy satisfies the engineering requirement.