The utility model discloses an overflow forming device for substrate glass. Overflow brick body, a drainage groove for draining molten glass is formed in the upper end of the overflow brick body; thelower end of the overflow brick body has a V-shaped structure; the outer sides of the two ends of the overflow brick body are each provided with a drainage baffle. one end of the drainage groove in the overflow brick body is a liquid inlet; a liquid inlet of the drainage groove is connected with an input port of the overflow brick molten glass conveying device; the utility model discloses a glasssubstrate forming process. molten glass is prevented from extending outwards along the two ends of the overflow brick body by utilizing the drainage baffles fixedly arranged at the two ends of the overflow brick body; therefore, the precision of the production size of the glass substrate is ensured, the structure is simple, clamping is performed without a cooling clamping mechanism, the operationis convenient, the structure is simple, the cost is reduced, the production fluctuation and the forming quality of the glass substrate are ensured, and more stable production is ensured.