We report a large mode volume unstable resonator working at 100 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 mJ is obtained at 100 Hz with slope efficiency of 36%. The pulse width is 15 ns, and the beam quality factor is M(x)(2)=3.2, M(y)(2)=2.8.