A method for developing the decoupled overall Jacobian of a planar symmetric 3-SPR parallel mechanism is presented by using matrix inversion technique,the 3-SPR parallel mechanism can be used to form the main body of a 5-DOF hybrid manipulator especially designed for high-speed machining and automatic assembling in the aircraft industry. Then a set of novel kinematic performance indices dimensionless,invariant with respect to the coordinate frame,and having a value ranged within [0,1] are developed by using the decoupled overall Jacobian. By investigating the influences of the homogeneous dimensionless parameters on these indices,the feasible domains of dimensionless design variables are obtained subject to the specified kinematic and geometric constraints,which not only meet the requirement of kinematic performance but also leave room for further modification.