Heat sink with the adiabatic groove is used to improve the asymmetry of the thermal focal length of laser-diode (LD) corner-pumped composite slab lasers. The numerical analysis indicates that the thermal focal length of two directions can be equal when the size of groove is 0.325 mm×2 mm×14 mm. The experimental results show that the difference of the thermal focal length of two directions decreases from 3.86 times to 2.19 times when the heat sink with the adiabatic groove is employed. Finally the calculation considering end effect and improving process precision of groove is put forward to further improve the asymmetry of thermal focal length.