A SCR（ Selective Catalytic Reduction） system was designed to decrease the NOx emission of diesel engine. By using Freescales＇ microcontroller MC9S12XS128,the hardware circuit of microcontroller unit（ MCU） control module,the power management module,the input adjustment module,the output driver module and the controller area network（ CAN） communication module was designed. The control strategy and procedure were formulated according to the principle of SCR system. Using a four-cylinder diesel engine as the sample engine,the European Stationary Cycle（ ESC） tests were made and the test results were compared with the original. The results showed that the brake specific emission of NOx was decreased to 1. 713 g /（ k W·h）,which was decreased by79. 44% compared to the orginal 8. 421 g /（ k W·h）. The maximum conversion efficiency of NOx could reach 92. 53% and the average conversion efficiency was nearly to 73. 21%. With the SCR system,the NOx emission of diesel engine could meet the limitation of China-V regulations. The electronic controlled system was qualified.