Disclosed are a maleimide-α-alkylstyrene-based, heat-resistant bulk terpolymer and a preparation process thereof. More specifically, disclosed are a bulk terpolymer, comprising 5-60 wt % of an N-substituted maleimide monomer, 10-70 wt % of an α-alkylstyrene monomer and 5-50 wt % of an unsaturated nitrile monomer, as well as a continuous bulk polymerization process for preparing the same. The disclosed bulk terpolymer has a weight-average molecular weight (Mw) of 70,000-300,000 and a glass transition temperature of 140-200° C., shows excellent high-temperature thermal stability and heat resistance and a remarkably low melt viscosity, and thus is excellent not only in processability, but also in productivity, processability, moldability and blendability, when it is blended with other resins. Also, the continuous bulk polymerization process is equipped with a devolatilizer and enables the bulk terpolymer to be produced at low cost and high efficiency.