Force and torque information is critical to enabling intelligent control of a robot in complex robotic applications. This paper presents a novel six-axis force/torque sensor based on optical fibre sensing for robotic applications in extreme environments with intense electromagnetic interference as well as explosive and inflammable materials. The designed sensor employs an elastic sensing element composed of 4 compliant beams and 4 elastic cross beams to convert the measured forces and torques to the strain on the surfaces of elastic cross beams, which is detected by 16 Fibre Bragg Gratings (FBGs). The strain is calculated by theoretical analysis using Timoshenko beam theory and validated by Finite Element Analysis (FEA). Working matrix of the sensor is constructed, which is associated with the relationships between the measured forces and torques and the wavelength shifts of FBGs. The sensitive coefficients obtained by theoretical analysis and FEA simulation are in good agreement, which indicates that the analytical method is accurate. The proposed six-axis force/torque sensor with low cost and high reliability has great potential in robotic applications in harsh industrial environments.