Lamb wave has many applications in non-destructive testing as its advantages of the capability for long-range testing from a single location. Electromagnetic ultrasonic technique also has many advantages, such as being free of couplant and preconditioning. So, electromagnetic ultrasonic Lamb wave possesses great superiority in inspection. However, Lamb wave's characteristic of multi-modes and dispersion, like other guided waves, is a significant problem for practical applications. In order to obtain clear signals that can be reliably interpreted, a method for optimizing excitation of electromagnetic ultrasonic Lamb wave has been studied. Based on the fundamental characteristic equation of Lamb wave, the exciting equation has been deduced and the exciting curves have been plotted to instruct the design of electromagnetic acoustic transducer (EMAT) for choosing reasonable operating points in the curves. The optimization method verified by experiments can effectively remove the effect of multi-modes and dispersive and the zero-order modes are of great engineering value in real applications.