The high-order sideband generation in a gently modulating optomechanical system, which consists of a standard optomechanical system driven by a mildly amplitude modulated field (MAMF) is investigated. It is found that the sideband generation can be effectively optimized by choosing an appropriate component number N of the MAMF, and this scheme can generate more and flatter high-order sidebands with the same input power compared with the conventional model. The efficiency of the sideband generation is much superior to the traditional scheme. The challenge for high-efficiently realizing a flat optical frequency comb based on optomechanical systems may be aided by this work.