Abstract: On the basis of numerical calculation methods, thepropagation characteristics of the high-order mode TE31,12 in the conventional open cavity with weakly tapered cross-section and improved cavity are simulated by numerical computation. Diffractive quality factors of open cavities with weakly tapered cross-section are also numerically computed. Then the influence of the curvature radius of the connection segment on the diffractive quality factor is investigated . The results show that the curvature radius of the connection segment is another factor that affects the diffractive quality factor and the larger the curvature radius of the connection segment is, the smaller the diffractive quality factor becomes. In order to prove the correctness of this conclusion, another improved cavity operating in the low-order mode TE0,1 is simulated by the simulation software HFSS, and the same conclusion is attained.