Foam glass was prepared with sintered process, using moderate amount of additives and the powder of waste stiff glass fiber as main material and its foaming mechanism was investigated. Its foaming mechanism is that some gas given off from foaming agents, is confined and therefore many small bubbles are formed in the brick of foam glass due to sintering. Because of softening of the glass and expansion of the bubbles at foaming temperature, the glass block begins to enlarge and to form foam glass. Proper bubble stabilizers, binders and increasing pressure in the brick forming process make sintering speed up and be in favor of the formation and stabilization of the bubbles.