High precision lithographic projected objective suffers from thermal aberrations due to energy absorption. Under off-axis illumination, the dipole mode and time-changing thermal astigmatism is outstanding, which cannot be corrected or compensated by traditional passive optical systems. Active optics using mechanical actuators to compensate the primary thermal astigmatism of deformed lens in a refractive lithographic objective system is proposed. The actuators are located on a refractive plate, and the finite element analysis is applied to study the deformation and aberration. The theoretical analysis by geometrical optics is made to prove the feasibility for compensation, and the influences of actuator locations, active force loads, actuator size and lens supporting conditions are considered. The results show that the active plate performs well to compensate primary astigmatism and primary 4- foil aberration under the optimized supporting conditions, which provides an idea for thermal aberration control in lithographic systems.