Using the 4-simplex and its dual-skeleton spin networks, a quantum transition of pure spacetime is described. The change of vertex volumes between successive steps of the transition of the three-dimensional space woven by spin networks is demonstrated. At the same time, a model of quantum flate fluctuation of the four-dimensional spacetime is given. Employing the excitations created when the loops on the edges of spin network pierce sarfaces, a 2 order symmerric tensor hab is obtained in three-dimensional space. Based on the viewpoint that the spacetime is excitation and transition system, the creation and the change mechanisms of the gravitational field h are investigated.