To solve the problem of hard connection in waveguide transmission line, some waveguide components will use flexible waveguide, but the use of flexible waveguide will bring about the increase of transmission line loss. Aiming to investigate its loss and electrical heating under real operating conditions, we built a test platform based on a resonant ring with a 13.4 dB power gain in the traveling wave of the resonant ring, which successfully achieves an equivalent power of 140 kW at the position of the antinode by means of two 2 kW power amplifiers. Based on the results of simulations and experiments, we optimized the design of the rectangular flexible waveguide and improved its structure and materials to better cope with the thermal deformation and stress under high power input. The optimized flexible waveguide's electrical and thermal performance is better than that of similar foreign products.