The number of electric vehicles (EVs) has increased sharply with the start of the 21st century. Although the automobile industry believes that drivetrains equipped with two-speed transmission can increase the energy efficiency of EVs by 4–5% and improve other vehicle performance indicators, most EVs still use fixed-speed ratio reducers. Does the two-speed transmission make sense? What is the latest development in the industry? What are the reasons this has not yet been popularized? This article attempts to analyze the current situation in-depth and provide a forecast of technology development trends. First, the authors analyze the advantages of the two-speed transmission from the energy consumption, economy, dynamic performance, and other indicators of EVs. Second, the topology optimization of the non-power-interruption two-speed transmission is introduced, and many efforts have been made to analyze the numerous studies both in academic research and industrial development. Then, a unified index for the necessity of adopting a two-speed transmission in EVs is given, which is verified by numerous data of different passenger cars and commercial vehicles. Finally, the authors predict that two-speed transmissions will be used first in sports cars, luxury cars, and electric logistics vehicles within several years, and then gradually penetrate ordinary family cars. •Analyze the advantages of the two-speed transmission of electric vehicles.•Introduce the topology optimization of two-speed transmission.•Give a unified index for the necessity of adopting a two-speed transmission.•Make predictions on the development trend of two-speed transmissions.