The directional excitation of surface plasmons play an important role in optical communication, biosensing, integrated circuit and nanolithography, etc. An asymmetric L-type single-slit structure which can excite surface plasmons directionally is proposed. Without changing structural parameters of the slit, both the unidirectional and the bidirectional excitations of surface plasmons can be realized by changing the incidence angle. The structure can also play a role in all-optical switch modulation. The machining process of the L-type single-slit is elaborated in detail. The slit structure is simulated by the COMSOL Multiphysics software. The results show that the asymmetric single-slit structure can realize the directional excitation of surface plasmons with a continuously adjustable extinction ratio from -16 dB to 15 dB. This single-slit structure occupies small area and is convenient for manipulation and machining, and it has guiding significance for the research on micro-nano plasmon optical devices.