To further study the blasting mechanism of slotted cartridge, blasting model for slotted cartridge with a charge decouple coefficient 2.0 was established by using coupled SPH-FEM algorithm in AUTODYN. The expansion process of the detonation product in the early stage of the explosion, the velocity of detonation product particles and the damage evolution of rock mass around borehole were analyzed. Results show that the detonation product particles can move forward at high speed due to lack the constraint of slotted tube in the slit direction. The maximum velocity of the detonation product particles can reach 4750 m·s^-1. The front-end product particles reach the borehole wall and the rock mass starts to be damaged at 3.5 μs. With the continuous expansion of detonation product，rock mass is further damaged in the slit direction. In the non-slit direction, the expansion of detonation product particles is hindered due to the restraint of the slotted tube. The maximum velocity of particles is only 800 m·s^-1. At the same time, the slotted tube driven by detonation products moves towards borehole wall slowly. The slotted tube reaches the borehole wall and rock mass starts to be damage at 12.6 μs. However, the small range of damage distribution is only formed, and the borehole wall maintains good integrity in non-slit direction.