The paper reports a method for monitoring the resin flow front in narrow channels using a fiber Bragg grating (FBG) sensor, which is used to study the resin penetration behavior in the joints between the pultrusion plates during the vacuum infusion process of the wind turbine blade pultrusion beam structure. The fiber optic grating sensor is pre-arranged on the side of the pultrusion plate near the joint, and the fiber is fixed at four points to form a bell-shaped curve, with the fiber optic grating located at the top of the curve. Four plates are stacked in pairs and spliced together, and a biaxial glass fiber fabric is placed between the upper and lower layers before pouring. The experiment shows that the center wavelength of the FBG is sensitive to the resin flowing into the joint from the bottom and top, providing a means for the study of the infusion process of composite materials containing joints.