With the development of aerospace technology, research of lightweight reflector is focusing on the mirrors with a larger radius to thickness ratio and higher lightweighting. However, when the surface accuracy of lightweight mirrors, such as PV values of below 0.1λ, mirror surface will obviously shows "imprinting effect" like the wave. The "imprinting effect" derives from the gravity and the pressure of polishing head has become a barrier for high-precision processing of lightweight reflector. It is mainly researched a new way that fills air into the inside of mirrors to resist the elastic deformation resulting from the pressure of polishing head. The simulation result shows that the immersion charge-method can eliminate the "imprinting effect", and the shape accuracy of mirrors is also much better than that caused by gravity.