In this paper, we measured the average lift generated by tethered flies and investigated the relationship among the average lift, geometry of wings, weights and other parameters. For this purpose, we proposed two different designs of force sensor. The first design can measure the dynamic force in one direction. It has wide bandwidth but suffers with low resolution. The second design has high resolution for average lift measurements and can simultaneously measure forces in two directions. However, its fundamental frequency is low and hence can’t measure dynamic force. We chose the second design in our experiment after comparing the pros and cons of these two designs. The experiment includes two parts: visual response and maximum lift measurement. In the visual response experiment, we studied the effects of vertically moving stripes on the fruit flies. Different ways of stripe movement have been tested. We found that the upward moving stripes can stimulate the fruit flies to flap their wings harder. Then we measured the lift of fruit flies subjected the influence of upward moving stripes.