This paper presents a classification of 3T1R parallel manipulators based on the wrench graph. By using the theory of reciprocal screws, the properties of the three-dimensional projective space, the wrench graph and the su-perbracket decomposition of Grassmann-Cayley algebra; six typical wrench graphs for 3T1R parallel manipulators are obtained along with their singularity conditions. Furthermore, the paper shows a way in which each of the obtained typical wrench graphs can be used in order to synthesize new 3T1R parallel manipulator architectures with known singularity conditions and with an understanding of their geometrical properties and assembly conditions.