A new concept of transforming a non-planar MUT to a planar MUT with an effective air gap is proposed in this paper. The air gap existing between the MUT and sensor usually make large error in measurement, but by sensing with a multiring sensor, the effect of the air gap can be eliminated. However, in the case of a non-planar MUT, the curved surface of MUTs make the multiring of the sensor sensed different thickness of air gap. Thus, the approach of above cannot be use to eliminate the frequency shift cause by the air gap, and this will cause a undesired error in the experiment. Therefore, a triple-ring complementary split-ring resonator (CSRR) is used to estimate the effective air gap that make the smallest difference of the three resonance frequencies. Finally, with the transforming equation, a non-planar porcine fat with 10 mm thickness, 20 mm radius of curvature and 0 mm air gap are transformed into an effective planar MUT with 0.646 mm planar air gap. The result compare to simulation only with an error of 0.067 mm.