To improve the sensitivity and dynamic performance, this paper presented a new type of flexible hot film sensor with a cavity underneath for thermal insulation. With FVM simulation and dynamic performance analysis, the structural parameters of the cavity were determined. The double side micro-fabrication processes of flexible hot film sensor with the cavity underneath were proposed. The static and dynamic performance of two kinds of sensor were studied in wind tunnel. Compared with the sensor without cavity, the new sensor shows 20% output increase with the same input and time constant drop from 400μs to 220μs. This means higher sensitivity and dynamic response speed were obtained.