Abstract: Abstract: The large production of the single diesel engine is the characteristic of China internal combustion engine industry. But the environmental pollution is more serious and the large quantity of the single diesel engine is one of the main pollution sources. The exhaust after-treatment is a good measure of the diesel engine to reduce the emissions. In order to improve the emission performance of single cylinder diesel engine through external purification, the one-body purification muffler was designed. The functions of the one-body purification muffler were noise elimination and emission purification. The internal velocity uniformity was one of the most important aspects of the structure design of the one-body purification muffler. Numerical simulation and performance test were adopted in the structure design and performance optimization of the one-body purification muffler to optimize the catalytic performance, acoustic performance, aerodynamic performance and structural performance. Numerical simulation was used to analyze the internal airflow velocity field of one-body purification muffler to optimize the internal structure. The reasonable internal structure of the one-body purification muffler could effectively improve the catalytic conversion efficiency and the service life. Performance test was used to analyze the catalytic performance, acoustic performance and aerodynamic performance of the one-body purification muffler. According to the results of the performance test, the internal structure of the one-body purification muffler was improved. The results of overall performance test showed that the test prototype with the third scheme of one-body purification muffler had the advantages of low power loss, low flow resistance and high noise elimination. The power loss of the one-body purification muffler based on the third scheme was 1.18%, which was reduced by 28.5% compared with the original muffler. The flow resistance of one-body purification muffler based on the third scheme was lower than the original muffler. With the third scheme of one-body purification muffler, the engine noise was 108.6 dB, which remained the same compared with the original muffler. The results of overall emission test showed that the test prototype with the third scheme of one-body purification muffler had the advantage of low exhaust emission value. The measured values of carbon monoxide (CO), hydrocarbon (HC) and particulate matter (PM) were 0.94, 0.46 and 0.4 g/kW·h respectively, which were reduced by 85.4%, 70.5% and 28.6% respectively, compared with the original muffler. But the measured value of nitrogen oxide (NOx) remained the same compared with the original muffler. Besides, the measured values of CO, HC and PM of the third scheme of one-body purification muffler were reduced by 21.0%, 25.8% and 7.0% respectively compared with the second scheme of one-body purification muffler because of the optimizations of the aerodynamic performance and the structural performance. After the endurance test, the exhaust emission values of the third scheme of one-body purification muffler still met the third phase limits. The third scheme of one-body purification muffler can improve the emission performance of single cylinder diesel engine through external purification effectively and optimize the catalytic performance, the acoustic performance, the aerodynamic performance and the structural performance. The research provides theoretical and technical references for the design of the one-body purification muffler of the single diesel engine.