The direct sintering process of coal fly ash for the preparation of glass-ceramics is the liquid-phase sintering process from non-densification to densification. When the temperature exceeds the densification temperature point, the porosity of glass-ceramics on the contrary increases and the pore diameter increases. This provides a basis for direct overfiring of porous glass-ceramics. Adding borax to coal fly ash can reduce the temperature of liquid phase formation, reducing the preparing temperature of porous glass-ceramics by overfiring, achieving the purpose of energy saving. The effects of borax on the structure, properties and sintering kinetics of porous glass-ceramics prepared from coal fly ash by overfiring were investigated. It is found that the introduction of B-O bond can change the network structure of non-crystalline vitreous in coal fly ash, reduce its melting temperature, promote the formation of liquid phase, and thus increase the porosity of porous glass-ceramics. When the adding amount of borax is up to 30%, the borax has been completely fused before destroy the non-crystalline vitreous network structure of coal fly ash by B-O to form liquid, which leading to the final phase of porous glass-ceramics is the same as coal fly ash. The bulk density, porosity and flexural strength of porous glass-ceramics are all affected by the liquid viscosity at high temperature, so the properties can be adjusted by adjusting the sintering temperature and the amount of borax added. In general, the addition of borax can promote the mass transfer in liquid phase, the formation of porous glass-ceramics, and the sintering activation energy also decreases gradually with the addition of borax. This paper provides a certain experimental basis for the preparation of porous glass-ceramics by direct overfiring of coal fly ash at low temperature without adding pore-forming agent, and provides a new possibility for the high-value resource utilization of coal fly ash.