The invention discloses a method for smelting Q235B steel through a CSP (compact strip production) line. The method comprises the steps that end point carbon, phosphorus, sulfur and temperature of molten steel in a converter are well controlled, carbon powder and silicon carbide are added into a steel ladle before tapping or alloying of the molten steel, red ladle tapping is conducted, and the amount of tapping slag and the tapping time are well controlled; lime and steel ladle slagging agents are added into the molten steel in the tapping process, and when tapping of one third of the molten steel is completed, alloying treatment is conducted by adding ferrosilicon and ferroaluminium into the molten steel; when tapping of three fourths of the molten steel is completed and the ferrosilicon and the ferroaluminium are added, calcium carbide is added into the molten steel subjected to alloying treatment; the molten steel is sent to an LF furnace to be subjected to refining, the content of steel ladle top slag and the content of the molten steel are slightly adjusted by adding the lime, the ferrosilicon, the calcium carbide and the steel ladle slagging agents into the molten steel in the refining process, the molten steel is not subjected to calcium adding treatment, and the molten steel is taken out of the LF furnace; the cooling water volume and a reasonable continuous casting cooling curve are controlled during molten steel continuous casting; and the technical parameters of heating and rolling process of a casting blank are the same as original technical parameters, and the quality of the casting blank and the quality of a rolled plate are ensured. The molten steel is not subjected to calcium adding treatment, so that the production cost is reduced.