Deficiencies are existed for currently formulae of mobility calculation for single loop mechanism. It doesn't reflect the constraint imposed to output components by link group. By analyzing the kinematic parameters of link groups, this paper proposes a new method to calculate the mobility of link group, which is the difference of the sum of the degree of freedoms (DOFs) of all the kinematic pairs and the rank of the kinematic parameters of link groups. Also the DOF can be obtained by the total DOFs of link groups and the rank of kinematic parameters of basis point of the output link. It can be concluded that the new method can not only express the move state of the output link, but the constraint imposed to output components by link group also be reflected. The research results can provide a reference for the structural analysis of the single loop.