As the SiC lightweight primary mirror in a 4 m photoelectric telescope has high specific stiffness and needs strict surface precision, this paper proposes an axial active support system based on the hydraulic whiffletree passive support for the primary mirror. The hydraulic whiffletree support system was used to support the mirror weight, and the active support was just output active forces to correct mirror surface errors, reduce the active force range and to increase the precision of active force. A 54-point axial support system and a 24-point lateral support system were optimized by the Finite Element Method(FEM), and the parameter of lateral force β was 0.5. With the hydraulic passive support system, the mirror surface deformation Root Mean Square(RMS) values from the gravity of the primary mirror are 37.8 nm and 82.9 nm respectively under horizontal and vertical states. After active correction, the mirror surface error RMS values are reduced to 12.0 nm and 9.8 nm accordingly. Analysis indicates that the mirror surface deformation of 4 m SiC lightweight primary mirror at different pitch angles is less than λ/30(λ=632.8 nm) and meets the requirements of surface errors.