The invention discloses a heat treatment process of a ship carbon-manganese steel casting, which is characterized by comprising the following steps of: (1) charging a ship carbon-manganese steel casting which contains not more than 0.23 percent of C, not more than 0.60 percent of Si, 0.7-1.60 percent of Mn, not more than 0.30 percent of Cu, not more than 0.30 percent of Cr, not more than 0.40 percent of Ni and not more than 0.15 percent of Mo into a large heat treatment furnace with the volume of 13m*11m*5m for carrying first normalizing treatment; heating to 920+/-20 DEG C at the temperaturerise speed of not more than 75 DEG C/h; insulating heat for 8-36 hours so that the tissue of the casting is completely austenitized; then discharging and cooling by air; when cooling below 700 DEG C in an hour, carrying out second normalizing treatment on the casting and heating to 830+/-20 DEG C still at the temperature rise speed of not more than 75 DEG C/h; insulating heat for 8-16 hours and naturally cooling below 600 DEG C along with the furnace to obtain a sorbite tissue; and then discharging and cooling to normal temperature by air. The process is stable and can meet the requirement onhigh toughness while the high-strength index of the casting is met.