Structure health monitoring of beam structures is important in civil and mechanical engineering. Lamb waves have the advantages of long propagation distance and low power loss, which are widely used in the field of nondestructive testing. However, Lamb waves generated via conventional methods usually contain multiple modes, and in many cases, the multi-mode Lamb waves cause difficulties in damage detection and localization. This research proposes a novel single-A0 mode Lamb wave generation technique; the actuator is attached on the side surface of a beam structure, and the sensor is attached on the top or bottom surface, which is perpendicular to the side surface, such that the single-A0 mode of the Lamb wave can be generated. Due to the mode conversion effect of the Lamb wave when it passes through a crack, the existence of the crack can be determined by evaluating the existence of the S0 mode. In addition, the crack can be localized accurately using the proposed algorithm. Numerical and experimental investigations are performed to verify the performance of the proposed method. Comparisons with other single-mode Lamb wave generation methods are also performed to validate the advantages of the proposed method.