A scheme of two-speed transmission is proposed based on an electric vehicle (EV) with a settled gear ratio. A new traction motor is chosen and a two-speed automatic gearbox without clutch is designed， and matching design between the motor and the two-speed gearbox is carried out. In order to improve operation efficiency of the motor， speed ratios of the two-speed gearbox are optimized aiming at lest energy consumption of the motor over the ECE drive cycle under the condition of ensuring the dynamic performance of the EV. A shifting strategy to guarantee high operation efficiency of the motor is drew up. The simulation results indicate that the energy consumption is decreased by 6.6% and the driving range is extended by 7.1% compared with the EV with a settled gear ratio.