We report a large mode volume unstable resonator working at $100\mathrm{Hz}$ . The resonator is combined with a variable reflectivity convex mirror and a high-reflective convex mirror. The cavity mirrors are used to compensate for the thermal lensing effect of the two Nd:YAG laser rods, which are pumped by high- energy high-repetition-rate diode lasers. A $90°$ quartz rotator is placed between the two laser rods to compensate for the thermal depolarization. In the experiment, output of as high as $388\mathrm{mJ}$ is obtained at $100\mathrm{Hz}$ with slope efficiency of 36%. The pulse width is $15\mathrm{ns}$ , and the beam quality factor is $M_x^2=3.2$ , $M_y^2=2.8$ .