Soft robot has the incomparable advantage in surgery and narrow space due to its flexibility, adaptation, etc. To realize the attitude sensing monitoring of soft actuator in small space (e.g., minimally invasive surgery), a flexible sensing method based on the implantable fiber Bragg grating (FBG) is proposed in this study. First, the FBG is implanted into the soft actuator. Then, under different bending states of soft actuator, the spectral, wavelength drift and curvature information of FBG are analyzed by experimental test. Threedimensional shape fitting and reconstruction of soft actuator are realized by cubic spline interpolation algorithm. Experimental results show that the error between the actuator′s actual bending angle and the reconstruction algorithm is less than 45%. The repeatability deviation index of sensor is less than 7%. The flexible sensing method based on implantable FBG can realize the attitude sensing and monitoring of soft robot, which has a broad application prospect in the field of medical surgery.