Abstract: Aiming at the problem of parallelism between plane mirrors and spherical mirrors in the plano-concave cavity resonator method, taking the K-band as an example and combining simulation and actual measurement, the impact of quasi-optical cavity parallelism on conductivity testing was studied. First, according to the different tilt directions and rotation radii of the sample, the impact of the tilt of the sample to be tested on the conductivity test was simulated and analyzed within the range of parallelism error ±1.2°; then, a quasi-precision experiment was carried out using the 24 GHz frequency point of the K-band as an example. Verification experiment of optical cavity parallelism error. The simulation results show that the conductivity test error increases with the increase of parallelism error. When the sample is coaxial with the spherical mirror, the parallelism error of ±1.2° corresponds to a conductivity test error that is basically within 20%; the actual measurement results of the K-band are basically consistent with the rules obtained by simulation. This study has certain reference significance for evaluating conductivity testing errors.