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This paper proposes a new method for exciting a specific mode of guided ultrasonic wave in a rail. The method requires the cross-sectional mode shape to determine excitation direction, excitation coefficient, and excitation position of each mode. The cross-sectional mode shape data of all modes are acquired by the semi-analytical finite element (SAFE) method. The optimum excitation direction is obtained by solving the Euclidean distance between different modes. According to the shape feature of a rail, the cross section of a rail is divided into three segments, including the rail base, the rail web, and the rail head. The excitation coefficients are obtained by analyzing vibration characteristics in these three segments. The covariance matrix of each mode at a specific frequency is established, and the result is used to obtain the best excitation point. The response to an excitation in the rail can then be computed by the SAFE method. It has been verified that the excitation method proposed in this paper can excite a specific ultrasonic-guided wave mode at the low frequency of 200 Hz and the high frequency of 35 kHz, respectively.