The invention provides a method and a system for improving the surface quality of continuous casting square and round billets and belongs to the technical field of steelmaking. The method comprises the following steps: mixing covering slag and carrier gas to obtain a gas and solid mixture; and heating the gas and solid mixture, spraying the mixture on the liquid level of molten steel and solidifying the molten steel to form continuous casting square and round billets, wherein the heat source for heating the gas and solid mixture comes from thermal radiation of the molten steel. The covering slag is heated by the heat of the molten steel, and the heat of the molten liquid is not absorbed largely when the heated covering slag is sprayed on the liquid level of the molten steel; meanwhile, theheated solid covering slag is sprayed on the liquid level of the molten liquid continuously and uniformly under the push action of the gas, so that the defect that the liquid level of the molten steel is non-uniform in local heat dissipation is avoided, the initial solidification state of the molten steel is improved, the cracking tendency of the molten liquid in the solidification process is reduced and the surface quality of the continuous casting square and round billets is improved.