The total supply capability（TSC） is a new important index to describe distribution networks while the tie-line is the key factor for TSC. A quantitative tie-lines contribution degree approach to the TSC is proposed. The effectiveness of tie-lines in distribution network based on the TSC is defined. And the calculation formulae of the effectiveness of both single tie-line and tie-lines combination are proposed based on the sensitivity of the network transfer capability（NTC）. In view of the increasing complexity of current distribution networks， a new method is proposed to reduce the number of tie-lines based on effectiveness calculation. Finally， a practical case study is given to illustrate and verify the effectiveness of the proposed indices and method. Some laws of the effectiveness of the distribution network tie-lines are summarized.