In order to accurately detect the angular momentum of light, an angular momentum detector based on surface plasmons was designed. The detector is small and simple, which can detect the incident light only by etching the designed nanoslit array on a gold film. When incident light illuminates the detector, the polarization of incident light can be distinguished according to the coupling direction of surface plasmons. Moreover, by measuring the light intensity of the outgoing port compared with the total intensity of the coupling direction port, the accurate topological charge of orbital angular momentum between －1 and +1 can be quantitatively calculated. The nano-structure detector can accurately identify the angular momentum of incident light, providing a new idea for the design and development of on-chip optical system.