A new method to fabricate continuous deep relief microstructure with enzyme-etching silver halide gelatin is proposed. Imaging principle of silver halide gelatin with enzyme etching and theoretical analysis of deep etching process are given, and the relationship between etching depth and exposure with absorption is solved. It shows that the nonlinearity of etching process is caused mainly by absorption of emulsion to light. The solving scheme to the nonlinearity and the optimum technique parameters of this process are presented to achieve the good linear relationship in relief depth of 4 μm by experiment. At last, the fabrication of micro cylindrical lens array with good profile has been achieved.