Blasting gas effect and rock crack growth have an important influence on rock breaking.The initial pressure of blasting gas under different dynamite installation structures and gas pressure’s variation with the displacement of stemming material are analyzed, from Newton’s second law the relationship of stemming material’s length and the time that it movements in hole under blasting gas pressure effect is derived. The rock crack growth is analyzed by using BCM model,the time that crack grows from hole wall to free surface along minimal resistance line is defined as rock breaking time, let the rock breaking time equal to movement time of stemming material in the hole, the calculation model for calculating stemming material optimal length is established. The model considers two aspects, which are the stemming material movement under blasting gas pressure effect and crack growth leading to rock breaking. The theoretical value is compared with empirical value and experimental value,and theoretical value is close to empirical value and experimental value, so the theoretical model has a definite reliability and can provide theoretical guidance for blasting engineering.