The measurement of the terminal operating force of the surgical robot is the key to achieve the precise control of the robot, which is crucial to ensure the safety of the surgical operation. In this paper, a method of measuring the three-dimensional force of soft robot terminal end based on fiber Bragg grating was proposed to meet the requirement of minimally invasive surgery. Based on the principle of fiber grating sensing, the sensing characteristics of fiber sensor embedded in soft robot were analyzed and a soft robot terminal force decoupling model based on linear calibration with least square method and nonlinear compensation with Bernstein polynomial was established. Then the relationship between the central wavelength shift of the fiber Bragg grating and the three-dimensional force at the end of the soft robot was studied. The results show that the average repeatability of FBG sensor is 1.5 pm. The measurement accuracy error of the end force in the three directions of XYZ is lower than 5% of the full range, and the residual distribution is mostly concentrated in the reliable range, with good repeatability. The proposed FBG decoupling algorithm provides an effective method for the precise measurement of the terminal force of soft robot in minimally invasive surgery, and has a promising application prospect in the measurement of the terminal force of soft robot in biomedicine.