The longitudinal distributions of beam in resonators always change with the focal length of thermal lens. Therefore, most of fundamental mode laser output from laser diode-side-pumped modules are usually obtained at given pump powers. The resonator model with thermal lens and the condition for fundamental mode operation are studied. The relation between the fundamental-mode spot at crystal and the focus of thermal lens is calculated, and the resonator is designed for dynamic fundamental-mode operation. At repletion rate of 10 kHz, the laser operates with dynamic fundamental mode and the average power can be obtained from threshold to 17.7 W, M2x=1.29 and M2y=1.20. Pulse duration is measured to be 200 ns and the peak power is 7.65 kW.