Owing to the increasing stringent regulations on diesel exhaust emissions, the performance requirements of exhaust gas treatment devices are becoming increasingly higher. However, the performance of after treatment systems efficiency at low temperatures still requires improvement. Herein, a new mixer is designed based on the traditional mixer to improve the performance of the engine at low speeds. The performance indicators such as the velocity uniformity, distribution of the uniformity of ammonia, wall-film temperature, and thickness distribution at the downstream pipe are studied. The exhaust backpressure of the traditional mixer and that of the new mixer are first compared based on computational fluid dynamics. Subsequently, the simulation results are verified by the experiments on the crystallisation area of different mixers at low speeds. The results show that the experimental and simulation results are consistent. The new mixer can effectively reduce the risk of crystallisation and improve exhaust emissions while ensuring a higher distribution of reducing agents. It provides an effective reference for the optimisation design of SCR (Selective catalytic reduction) structures.