In this study, a dual-channel probe type 81° tilted fiber grating (81°TFG) sensor is proposed and used for the evaluation of alpha-fetoprotein (AFP) immunoassays. First, the end face of 81°TFG is plated with a silver film to form the reflective type 81°TFG, and a dual-channel sensor is constructed by a 2×2 coupler and two 81°TFG probes. Then, large-size AuNs and GO are modified on the grating surface, and AFP monoclonal antibodies are used as the specific recognition unit to modify the grating surface to achieve a dual-channel probe type 81°TFG local surface plasmon resonance (LSPR) AFP immunosensor. Finally, the immune detection experiment is completed in a complex serum environment. The results demonstrated that the detection limit of the sensor for the mass concentration of AFP antigen is within the range of 1--10 pg/mL, the detection saturation point and sensitivity are ~200 ng/mL and ~0.155 nm/log(pg/mL), respectively, it has a satisfactory specific response to the serum of liver cancer patients is observed. Compared with the single-channel transmission type 81°TFG sensor, the sensor could simultaneously control the detection of positive and negative samples of the target molecules; therefore, enhancing the reliability in clinical detection applications, and the operation is simpler and the application potential is greater.