This paper presents a three-dimensional force sensor based on fiber Bragg grating (FBG) for robot plantar force measuring. A classical Maltese-cross beam with multiplexed FBGs has been designed for three-dimensional force sensing. Strain distribution characteristics and dynamic performance of the Maltese-cross elastomer have been investigated by using finite element analysis (FEA). Through ingenious design, three-dimensional forces of Fx, Fy, and Fz have been measured by only five sensitive elements of FBG, meanwhile, decoupling and temperature compensation have also been realized, which greatly reduces the number of sensitive element compared with the traditional resistance strain gauge based multi-axis force sensor. Comprehensive performance test has been carried out, and the experimental results demonstrate that the sensor possesses good linearity, weak coupling, and creep resistance.