Accurate measurement of three-axial force is essential for dexterous manipulation of grippers. Capacitive sensor is one of the most effective ways to achieve tactile sensing. However, current researches mainly focus on measurement of normal forces. In order to improve the accuracy of measuring tangential force and further reduce the size of sensors, a highly-decoupling triangular capacitive sensor structure, which is based on floating comb electrodes, is proposed in this paper. An analytical model that combines normal force and tangential force together is established and corresponding formulas are put forward. The model is validated by Finite Element Analysis (FEA). The experimental results show that the proposed structure achieves high decoupling of tangential forces with a smaller size compared with previous structures.