The paper makes an attempt to unify the design of linear consensus protocol for multi-agent controllability. The problem is studied under leader-follower framework. We first show that protocols can be respectively designed for a high-order and single-integrator multi-agent system under which the controllability is proved to be determined uniquely by topology structure of the communication graph. Then this observation is further proved to be hold for generic system. To this end, an equivalent transformation is introduced and an approach is presented to design protocols for the controllability of generic system. Finally the controllability is evaluated in a unified way with respect to the dimension of system, the network topology structure and the design of protocols, which reveals that controllability can be regarded to a certain degree as an intrinsic property of multi-agent systems. It is also proved that the controllability is equivalent to that of a simplified subgraph of the original communication graph.