A frequency multiplication conversion apparatus for realizing temperature regulation of large-caliber crystals in a vacuum environment is disclosed, relating to the frequency multiplication conversion apparatus for the large-caliber crystals, and in particular to the frequency multiplication conversion apparatus for realizing temperature regulation of the large-caliber crystals in the vacuum environment. The frequency multiplication conversion apparatus aims to solve the problems that the existing heating apparatus cannot realize accurate temperature control of large-caliber crystals and cannot maintain higher temperature surface uniformity of the crystals. A first box comprises a first inner box and a first outer box; the first inner box is sleeved with the first outer box; a first water inlet groove, a first water return groove and a plurality of first water diversion grooves are processed in the outer surface of the first inner box; the first water inlet groove and the first water return groove are arranged in parallel in the length direction of the first inner box; the multiple first water diversion grooves are distributed in parallel around the outer side wall of the first inner box; and one end of each first water diversion groove is communicated with the first water inlet groove while the other end of each water diversion groove is communicated with the first water return groove. The frequency multiplication conversion apparatus is used for realizing frequency multiplication conversion of high-throughput large-caliber lasers.