As a new scheme of the distribution networks, the DC distribution network should be compared in detail with the AC distribution network in planning and design stage. Mean while, the voltage sags problems are also dominated in resent years. Under this circumstance, this paper studies the effect of DC distribution network which can relieve voltage sags occurs of the sensitive customers. Then, the life cycle cost (LCC) model of AC/DC distribution network is established by considering the economic and reliability impacts. It is shown that, in the case of 50% DC loads, even the DC distribution network has more initial cost and lower reliability, the LCC of DC distribution networks is better due to its ability to mitigation voltage sags and less loss. At last, the optimization method of AC/DC distribution network under different DC load ratios and power electronic device failure rate is proposed.