The injection angles of Yb∶YAG slab are calculated and the configuration of laser amplification is optimized. The advantages of the reflective image transfer system are compared and analyzed, and the spherical aberration of the image transfer system is calculated. The results show that the spherical aberration is significantly decreased by the reflective image transfer system, which improves the beam quality of the amplified laser. Based on single Yb∶YAG slab, a three-passes master oscillator power amplifier system is developed at room temperature. By using the reflective image transfer system and matching the near-filed intensity between pump laser and seed laser, an output power of 7.13 kW and a beam quality of 2 times diffraction limit are achieved without the active optical correction system.