A method for modeling the longitudinally non-uniform thermal lensing is proposed, the laser rod is divided into N equivalent segments longitudinally for calculations, each can be dealt as lens-like medium, so both the forward and backward transmission matrix of longitudinally non-uniform thermal lensing can be worked out. A quantitative analysis of the fundamental-mode distribution in a plane-parallel cavity is made using transmission matrix. The effect of the dividing number N on waist radius of cavity mode is analyzed, results show waist radius changes with the number N, when N is larger than a certain value, waist radius will tend to stabilization and can get closest to realities. The smallest values of N for the waist radius getting to stabilization are discussed with the variations of the pump power, average pump radius and rare earth ion doping concentration, individually.