The thermal effect of gain medium is an important factor to control the high power and high beam quality of the grazing incidence slab laser. The Nd∶YAG and sapphire crystal bonding method is used to study the thermal effect of grazing incidence slab laser. The temperature field distribution and thermal focal length of gain media of Nd∶YAG and sapphire bonding crystal are simulated theoretically. The output characteristics of gain media of Nd∶YAG and sapphire bonding crystal and single Nd∶YAG crystal are compared experimentally. The maximum stable output power of the bonding crystal is 26% higher than that of the single Nd∶YAG crystal, under the same condition of cavity length and pump free oscillation. When the pump power is 44 W, the thermal focal length of bonding crystal increased by 69% with respect to the single Nd∶YAG crystal. The beam quality factors along the width and thickness direction of bonding crystal are M2x=1.84 and M2y=2.29, respectively. The beam quality factors along the width and thickness direction of single Nd∶YAG crystal are M2x=2.92 and M2y=4.38, respectively. The theoretical analysis and experimental results show that the Nd∶YAG and sapphire bonding crystals can reduce the thermal effect of gain medium in grazing incidence slab laser, which is beneficial to improve the output power and beam quality.