The lithography etching process was developed to make the continuous surface micro-lens structure on a self-supporting polyimide (PI) thin film. The photo-resist sculpture structures were obtained by means of moving mask exposure technology firstly, and then they were transferred onto a PI thin film with high-fidelity. While the key point of our process is how to achieve equal ratio etching. The affection of optimization of etching gas composition、ratio and flow to the equal ratio etching was studied respectively. Under the condition of some process parameters are fixed (such as the RF power, chamber pressure and self-bias), when the content of O2 was a constant in the mixing work gas of O2+SF6, with the percentage of SF6 increasing to 13.04%, and the total flow was the optimal value of 46sccm, which makes the etching rate is 136nm/min and the etching ratio is 1∶1.02. The anisotropic etching with the 2.3% etching error was obtained finally. It has proved that the micro-lens array fabricated on a PI thin film has the same optical performance comparing to a micro-lens array on a quartz substrate, by using the focal spot testing setup in our lab.