In laser resonator, temperature gradient distribution exists in the edge cooling laser crystal, which makes laser crystal be equivalent to a thermal lens. The laser beam diameter changes with the pump power and the laser resonator structure due to the thermal lens effect, which influences the output power. Rate equations considering the variation of the laser beam diameter are presented and numerical methods are used to solve them. The theoretical results show that the relationship between the output power and the pump power depends on the laser cavity structure. Experiments are taken with 940 nm end pumped Yb∶YAG with emission of 1030 nm. Experimental results show that output power is influenced by the cavity structure. The experimental data agree well with the stimulation results.