The invention provides a multimode-single-mode structured optical fiber surface plasma resonance sensor and a detection method thereof. The sensor comprises a broad-band light source, an optical isolator, an optical fiber coupler, an optical circulator, a multimode fiber, a coating sensing unit, a spectrograph and a computer. Light emitted by the broad-band light source is transmitted to the core of the multimode fiber, and at the junction of the multimode fiber and a single-mode fiber, because the core diameter of the single-mode fiber is far smaller than that of the multimode fiber, an optical signal in the core of the multimode fiber is partially leaked to the cladding of the single-mode fiber, and surface plasma resonance effect is generated at the sensing unit so as to carry out online detection on external substances. According to the invention, light leaking to the cladding of the single-mode fiber is sufficiently used for detection, treatment on the cladding is avoided, influence of the cladding of the fiber on the sensor is eliminated, the detection sensitivity of the sensor is improved, the making difficulty of the sensor is reduced, and compared with a sensor with a multimode-single-mode-multimode structure, the light transmission efficiency of the sensor is improved, and detection for a weak signal is realized.