The carrying capacity of a mechanism is closely related to its line vector force. A thorough isotropic analysis of line vector forces was presented. The results showed that the isotropic condition can be satisfied when the line vector forces were evenly distributed on a conical surface with a conetop angle of 109.472°. In addition, since few three degreesoffreedom (DOFs) translational redundantly actuated parallel manipulators were reported, by combining the isotropic line vector forces, a method for the type synthesis of 3DOF, redundantly actuated translational parallel mechanisms (PMs) was proposed, in which the arrangement of the active joints was taken into account in advance. Using this method, firstly a kinematic chain with a twist system reciprocal to both the constraint and actuation wrenches was constructed, and then the active joint reciprocal to the constraint wrenches but not to the actuation wrench was constructed. Thus, a series of typical redundantly actuated PMs with isotropic actuation forces were obtained. Finally, the 4P〖TXX-〗RRR PM was analyzed as an example, and the results showed that the isotropy of the loadcarrying capacity can always be satisfied during its movement, because the condition of force Jacobian matrix was always equal to one, which to some extent verified the correctness of the isotropic analysis and the synthesis method proposed in this research. For this method, it not only satisfied the expected requirements, but also meant that the active joints no longer needed to be selected, and the rationality of the selection did not need to be considered, which provided a new method for the type synthesis of parallel manipulators.