Abstract: In order to study the thermal focal length in LD corner-pumped composite slab lasers more clearly,the theoretical temperature distribution and the thermal focal length in a LD corner-pumped Nd:YAG/YAG composite slab were calculated with the finite-element method.Moreover,the thermal focal length of the slab was measured with CCD under non-lasing conditions.A comparison between the theoretical results and the experimental results was made.The result showed that the experimental thermal focal length transformation was in accordance with the theoretical computation.