The diesel premixed low-temperature combustion mode avoids the generation of thick mixture and the high temperature region in which a great amount of NOx and PM generates. It makes a significant reduction in the emissions of both NOx and PM available at the same time. However, with the quantity of pre-injection increases and the injection time advances, the emission of HC increases significantly, which causes a decrease in the combustion efficiency. Studies have shown that the flame quench caused by too thick or too lean mixture and the oil film on the chamber is the main source for the emission of HC. As a result, understanding the mechanism of atomization and evaporation of the fuel and the formation of the mixture makes significant sense. This paper focuses on the mixture formation process. And the methods of testing the distribution of the mixture, the influential factors and control methods are studied. The results show that single-stage fuel injection leads to much fuel to be adhered on the cylinder wall and forms a thick mixture zone near the wall, which affects the overall level of fuel atomization and uniformity in cylinder. In addition, higher injection pressure, smaller injection pulse and higher cylinder temperature are conducive to the evaporation of diesel and they can help reduce the regions in which the mixture is too thick or too thin. They are important means to improve unburned hydrocarbon (UHC) emissions of the diesel premixed low-temperature combustion.