A cover layer of composite board with air purification function is characterized in composed of 0.5-10% of tourmaline powder, 40-50% of magnesium oxide, 25-30% of magnesium chloride, 2-30% of calcium carbonate and water 4-8% of, referring mass percentage. Compared with the prior art, the invention adds tourmaline powder into formula to generate calcum carbonate to decompose the organic gases as formaldehyde and phenyl in air and ammonia, and disinfect bacterial and mould in air, thereby purifying air.