Flapping-wing micro air vehicles (FWMAVS) are subject to lift, drag and inertial loads of violent changes while fluttering at high frequencies, which seriously affect the flight performance and flight life. This paper establishes a mathematical model of wing lift, drag and inertial forces during the FWMAVs fluttering based on the analysis of movement and force conditions of the wing. A multi-objective optimization model targeting at improving the distribution of loads in the time domain is then proposed and solved by using NSGA-II algorithm in Matlab environment. After all the work, the Pareto optimal solution set of the hovering state is obtained. Comparison and analysis of the data show that after the optimization, the peaks of lift and inertial forces decline significantly and the load distribution is obviously improved, which shows that results of this optimization method meet the design requirements.