Disclosed are a heat-resistant SAN resin, a method of preparing the same, and a heat-resistant ABS resin composition comprising the same. More particularly, a heat-resistant SAN resin comprising 60 to 75 wt % of α-methylstyrene, 25 to 35 wt % of a vinyl cyan compound, and 0 to 10 wt % of an aromatic vinyl compound (except for α-methylstyrene), the heat-resistant SAN resin having a degree of branching of 0.40 to 0.60, a molecular weight of 90,000 to 150,000 g/mol, and a glass transition temperature (Tg) of 124 to 140° C., a method of preparing the same, and a heat-resistant ABS resin composition comprising the same are disclosed.