In the production of polyurethane sandwich insulation board, void defects are easy to form in the polyurethane foam layer due to uneven foaming, and it directly affects the heat conservation effect. For this reason, a rapid detection method implementing electromagnetic ultrasonic SH guided wave inspection from the outside of the metal board is proposed. Through theoretical analysis, analytical derivation, simulation calculation and experiment verification, the relationship between the transmission reflection coefficient ratio of the SH guided wave in polyurethane sandwich insulation board and the excitation parameters is established. It is found that when the magnet spacing is 4. 5 mm and the incident angle of the SH0 guided wave is about 75°, the best detection effect can be achieved. Furtherly, it is verified that the amplitude of the received signal of the SH0 guided wave has linear relationship with both the defect area and defect depth basically. On this basis, a defect equivalent size evaluation method that could be applied in actual inspection is proposed. With the evaluation formula, the problem of defect signal evaluation fluctuation caused by the change of the distance between the receiving and transmitting probes can be eliminated, which has a good instruction for the later realization of defect location.