In a three-stack side-pumped laser, the gaps between two arrays for cooling make the pump light nonuniform and affect the thermal uniformity in the crystal. Based on the light intensity equation and the distribution characteristic of hot source, numerical method for thermal lens focus is proposed. With the total pumping power unchanged, the effects of dimensional duty factor of arrays and the distance between LD and the center of the crystal on the thermal lens focus are numerically calculated. The thermal lens focus increases along with the increasing array dimensional duty factor and the pumping distance. The result indicates that the evener the pump light distributes along the axis, the smaller the thermal lens affects. It can furnishe evidences for making choice and improving pump resources when axial thermal uniformity is concerned.