With the widespread use of mixed emulsion explosives, there exists the situation that the stemming materials penetrate into emulsion explosives in open-pit blasting construction, which indirectly changes the stemming length and reduces the blasting effect. To study the influence of stemming on mixed emulsion explosives, stemming simulation tests, numerical simulation tests and field blasting tests were carried out successively. Firstly, four kinds of PVC pipes with common hole diameters were used to simulate the blasting holes, and the rock chips were used to simulate the stemming materials. They were used to systematically study the infiltration of rock chips into the explosives on top of the explosive column during the filling process of stemming. Secondly, the numerical simulations of single-hole blasting were conducted with the LS-DYNA software, and the influence of the stemming mixture at the top of the explosive column on the explosive power was investigated by observing the changes of stress values at the measurement points. Finally, the stemming filling process was improved by setting physical isolation during the charging process based on the bench blasting in the Jinduicheng open pit mine. And the effect of stemming on the mixed emulsion was analyzed by comparing the blasting effect before and after the physical isolation. The results show that the phenomenon of explosive overflow appears on the top of the explosive column in the explosive charging process. Then, the rock chips gradually infiltrate into the emulsion explosive due to the influence of gravity after the charging is completed, and the infiltration length of rock chips increases with the increase of hole diameter in the same time. The numerical simulation results show that the stresses of different monitoring points have decreased, and the infiltration of stemming reduces the explosive power of the top explosive. The fragmentation analysis of the top rock after improvements shows that the main fragment size distribution of rocks decreases from 20~40 cm to 0~20 cm, and the proportion of rocks over 60 cm decrease from 6.13% to 1.81%, compared with the conventional charging process. In summary, the stemming has a significant impact on the power of emulsion explosive and blasting effect, and it can effectively improve the blasting effect by taking physical isolation to separate the stemming and explosive.