Rigid wing is a kind of wing model when making the experiment of high lift mechanism in insect flight and the research of numerical simulation. In this model, we neglect the wing distortion and regard insect wing as a rigid sheet. On the contrary, flexible wing is a wing model that differs from the rigid wing when analyzing the flight theory of the bionics flapping aerocraft. In this model, we think that the wing distortion has a great effect to lift force and regard insect wing as a flexible sheet. When considering the insect wing as a flexible wing, the force analysis of insect flying has a big difference from the traditional analysis to the rigid wing. Based on the viewpoint of the flexible wing, this paper researches some key points on the flight force of the insect wing such as wing structure, the measures of the wing distortion, the influence from the wing distortion for the lead airstreams angle and the lift coefficient, the reciprocity of the wing distortion and the vortex, Magnus Effect and Wake Capture. A clear understanding toward these problems is a help to know the reason why the flexible wing is advanced to the rigid wing and why the bionics wing just need to make a simple rhythmic locomotion. The discussion in this paper will benefit the future research to the bionics flapping aerocraft with flexible wing.