To enhance the guided wave amplitude excited by the single-magnet electromagnetic acoustic transducer (EMAT), a simple methodology using a semi-closed yoke structure is proposed in this study, which is based on both simulation and experiment analyses. The proposed EMAT structure consists of one rectangular-shaped magnet, a semi-closed yoke, a meander-line coil, and a metal sample. Through simulation analyses, three different possible yoke structures are compared, and the feasibility of the semi-closed yoke structure for waveform enhancement is confirmed. The parameters of the semi-closed yoke structure are carefully studied, including side plate spacing, top plate spacing, plate thickness, and relative magnetic permeability of the material used. Therefore, their optimized combination achieves the highest possible waveform amplitude. Compared with the original yoke-free solution, the semi-enclosed yoke EMAT with optimized parameters achieves an excitation efficiency improvement of nearly 50% in simulation. Accordingly, the optimized EMAT units are manufactured. Experimental results on a 1 mm thick aluminum plate show that with a lift-off distance of 2 mm, the 10-cycle 1 MHz S0-Lamb wave excited by the proposed solution exhibits a 40.42% amplitude enhancement compared to the original design. When the lift-off distance is 6 mm, the improvement is 31.32%. Then, a 5-cycle 415 kHz A0-Lamb wave is implemented on a 2 mm thick aluminum plate, and the amplitude enhancement brought by the proposed scheme is more than 30% at different lift-off distances. Hence, the general applicability of the proposed method is verified. The used yoke is made of common cold-rolled steel sheet (SPCC), which is readily available and suitable for practical applications.