With the rapid development of electrical network construction, it is an urgent need to solve the problem of reasonably planning AC and DC transmission voltage class sequence in the transmission network. As the major object of study, the applicable range of AC and DC transmission mode and their voltage class sequence is explored in this paper. Firstly, the fast establishing method of AC is proposed under the given transmission requirement, and the applicable range of EHV and UHV transmission is summarized after the simulations under different transmission scenarios. On this basis, combining the economical transmission distances of DC voltage class sequence, which has been researched before, the suitable AC and DC transmission schemes can be selected under the given transmission scenario. Then, based on the AC and DC transmission characteristics, the comprehensive evaluation index system of AC and DC transmission schemes is established, which has considered multiple factors, including power transmission characteristics, reliability, economy and so on. Meanwhile, a grey comprehensive optimal selection method of AC and DC transmission schemes is put forward. Finally, in several groups of typical transmission scenarios, the most economical AC and DC transmission schemes are established, and then using the grey comprehensive optimal selection method and the evaluation index system, the optimal one is selected comprehensively. Through the simulations, the applicable range of AC and DC transmission voltage class sequence is summarized.