A high repitition rate mode-locked Nd: YAG laser is reported. Pumping induced thermal lens effect in resonator is studied and the conditions for thermally stable resonator is analyzed.Two configurations of thermally stable resonators have been designed and the experimental results given. The maximum repetition rate of laser is 30pps (TEM00 mode), the energy of envelope is about 10mj, the divergence of beam is about 0.5mr and the minimum pulse width(FWHM)is 21ps.