Abstract: NO_x measurements downstream of diesel engine aftertreatments respectively with box shape and barrel shape were carried out to verify whether the NO_x sensor meets accuracy requirements of the China Ⅵ emission standard. The experimental results show that the NO_x measurement accuracy is achieved when the NO_x sensor is arranged on the rear surface of the box shaped aftertreatment,while that is not achieved when the NO_x sensor is located on the rear cone of the barrel shaped aftertreatment. The simulation results for the barrel shaped aftertreatment show that the ammonia concentration distributed on the rear cone surface has a uniformity degree of 0.957,but the ammonia concentration in the central location of the rear cone surface is locally high,which does not meet the NO_x measurement accuracy requirements. The ammonia concentration distribution in the straight section of the tail pipe has a uniformity degree of greater than 0.970,and the local area with high ammonia concentration is gradually decreased with the increase of the length of the tail pipe. Sensor location test results show that the NO_x measurement accuracy can be achieved when the sensor location is moved from the tapered surface to the straight pipe section of the tail pipe and has the minimum distance of 190 mm to the cone of the barrel shaped aftertreatment. A post-cavity mixer was then designed and is found that it can increase the ammonia concentration uniformity to 0.996 when measured at the current NO_x sensor location and there is no local high ammonia concentration problem. The issue of the NO_x measurement accuracy downstream of the exhaust aftertreatment is solved.