The invention provides a first-tank flocculated-flow-preventing method used for aluminum-killed steel produced by using a thin and medium slab continuous casting machine. According to the invention, a first-tank desulphurization rate of an LF furnace is higher than 80%; weak argon stirring is adopted during a wire feeding process, a stirring time is controlled at 5-10min, and a calcium weight percentage in molten steel is no smaller than 0.0020%; a tundish baking temperature is no lower than 1100 DEG C; a submerged entry nozzle baking temperature is no lower than 1100 DEG C; a time interval since the tundish baking device is raised until casting is started is shorter than 5min; argon blowing operation is carried out before an upper nozzle is installed; before the tundish is cast, argon is delivered in, and reductive atmosphere in the tundish is maintained. With the method, inclusions can be well absorbed by top slag, and can sufficiently float, such that the generation of flocculated flow can be controlled. With the method, a first-tank flocculated flow occurrence rate when aluminum-killed steel produced by using the thin and medium slab continuous casting machine is treated by using the LF furnace is reduced from 50% to 2%; a casting interruption accident caused by flocculated flow is avoided; and the stability of a steelmaking production process is ensured.