The invention discloses a continuous casting method for controlling the MnS inclusion size in an alloy steel square bloom. In the continuous casting process, the molten steel superheat degree is controlled to be 20-30 DEG C; the drawing speed is controlled to be 0.5-0.6 m/min; the crystallizer water volume is controlled to be 170-180 L/s, and the inlet and outlet temperature difference is controlled to be 4.2-4.8 DEG C; the crystallizer electromagnetic stirring current is controlled to be 200-240 A, and the stirring frequency is controlled to be 2.2-3.0 Hz; the tail end electromagnetic stirring current is controlled to be 150-170 A, and the stirring frequency is controlled to be 8-10 Hz; and the secondary cooling specific water flow is controlled to be 0.22-0.25 L/kg, and secondary coolingwater proportions of all zones are 0.35, 0.4 and 0.25 respectively. The method aims at medium-low-carbon alloy structural steel based on the continuous casting process element segregation control principle, by controlling the continuous casting technique, the MnS distribution state of the alloy structural steel square bloom is effectively improved, and the method has the characteristics of beingsimple in technique, good in effect and the like.