Abstract: The effects of the misalignment between the inner-rod axis and the outer-wall axis in an open coaxial resonator are studied by the software CST microwave studio. Results show that the field amplitude and S parameter decrease with the oblique misalignment angle increase. But the effect is little, and the coaxial waveguide can work normally. While the oblique misalignment angle is 1°, the field profile of TEM mode at the coaxial waveguide’s port is much different from the profile with very small oblique misalignment angle, the device might not work normally.