To study the lift generation mechanism and energy expenditure of clap and fling motion for insect flight, an unstructured mesh finite volume ALE method is developed. To solve mesh deformation problems during clap and fling motion, we adopt two mesh modification techniques. A 2D elliptical wing model is used to validate our solver by comparing the solutions to the already published numerical and experimental results. We analyzed the lift generation mechanism of clap and fling motion. The paper also released the comparison of Cl and energy expenditure between the single wing hovering motion and clap & fling. Conclusions have been made: the wake capture and delayed stall contribute to the lift generation while the rapid pitch mechanism decreases the lift. Clap and fling motion generate less lift but consume less energy, which means that it can generate more lift as consuming the same energy compared to single hovering motion by quantitative analysis.