The invention discloses a brazing method of a water-load isolation ceramic chip and a brazing fixture. The ceramic chip and a square-wave guide tube are welded. The ceramic chip is arranged inside the square-wave guide tube. Locating pieces are uniformly arranged on the periphery of the square-wave guide tube. Molybdenum wires twine around surfaces of the locating pieces so as to fasten the locating pieces. A brazing solder is placed on the ceramic chip. The components are put into a hydrogen furnace, and thermal insulation is carried out at 780+/-5 DEG C for 1-3 min so as to finish the brazing. According to the invention, fit clearance between the ceramic chip and the square-wave guide tube is accurately controlled, the locating pieces are arranged on the outside of the thin wall of the square-wave guide tube, and the locating pieces are fastened with the molybdenum wires. Then, during brazing of the square-wave guide tube and the square ceramic chip, expansion amount of the oxygen-free copper square-wave guide tube is greatly reduced, it is ensured that infiltration of the solder between the ceramic chip and the square-wave guide tube is good, and brazing quality is guaranteed. In addition, air leakage is avoided when water is loaded at more than two atmospheres of pressure; vacuum tightness of a microwave window can be guaranteed; and performance of a microwave vacuum device is further guaranteed.