The invention discloses a method for producing peritectic steel by using a medium-thin slab continuous caster. The method comprises the following steps: converter steelmaking, external refining of an LF furnace, and continuous casting of the medium-thin slab continuous caster. According to the invention, the entire process of converter steelmaking, LF furnace refining and continuous casting of the medium-thin slab continuous caster is controlled, so that the crystallizer liquid level fluctuation range is controlled within +/-3 mm; the deoxidization and alloying of the steelmaking process are controlled to enable the steel inclusions to be mainly spessartine with low melting point and strong plasticity; in the refining process, the secondary oxidation of molten steel is reduced, and an argon system is designed reasonably to ensure the maximum flotation of the inclusions; during continuous casting, the pouring is protected, the secondary oxidation of molten steel is reduced, the properties of the inclusions in the molten steel are changed, and the pourability is improved, so that the mass normal stable production of peritectic steels by using the medium-thin slab continuous caster is realized.