The invention provides a three-dimensional force sensor used for well drilling and based on a six-branch-chain parallel mechanism. The three-dimensional force sensor mainly comprises a stressing table, an end cover, a base and a sensing mechanism, wherein the end cover is mounted at an upper end port of the base, an upper cavity is formed in the base, the sensing mechanism is mounted in the upper cavity and comprises a movable table, a fixing table and three sensing arms with the same structure features, a plurality of force sensors are arranged on the sensing arms, the stressing table transfers the stressing state to each force sensor, and each force sensor senses the stressing state to obtain detection data. The three-dimensional force sensor has the benefits as follows: the parallel force sensors of the sensing mechanism are taken as elastic elements, the temperature drift problem of strain gauges is solved, and besides, the coupling problem that measurement parameters cannot be decoupled is effectively solved by means of the parallel structure. The measurement error is reduced, and detection is accurate. The force sensor adopts redundant mechanism design, and the reliability of the sensor is greatly improved due to uniform stressing by the three sensing arms.