Analyze the explanation of inertial and inertial reference system by Galileo, Newton, Lange and so on, remove the unreasonable content, analyze the applicable conditions of the laws of Newtonian mechanics, think that: Inertial is the object’s ability to keep the former motion state. The object will keep its former motion state without new forces acting on it. And the object’s former motion state is variable motion. Inertial reference system is the reference system which can separate the motion of the object being studied under specific force out alone from the former variable motion state. If the force f acts on the object being studied rather than the reference object, and the other acting forces besides f don’t cause relative motion of them, or may cause relative motion but can be removed by reasonable ways, then under this condition observe and study the motion law of the object being studied under the force f, the reference system fixed on the reference object is inertial reference system.