Abstract: Studied here in the paper is the influence by the heat treatment process on the residual austenite content and the mechanical properties of the expandable casing. As a result, it is revealed that compared with the heat treatment process of quenching + hi-temperature annealing, employment of the heat treatment process of quenching + critical section normalization + hi-temperature annealing is capable of bringing about more residual austenite structure, and thus proper residual austenite content gets the comprehensive mechanical properties of the steel improved, the strength ratio reduced, the plasticity and toughness of the expandable casing enhanced, and the uniform elongation increased from 12% up to 23% so as to meet the requirement for the expansion.