Investigated radius is an important parameter in oil and gas field development.In multiple composite reservoir, the investigation radius cannot be calculated by the existing software accurately.In view of this problem, this paper deduced the formula for calculating investigation radius and formation pressure distribution in dual composite reservoir, by using the virtual investigated radius method.On this basis, the investigation radius and unstable pressure analytic expression were derived in multiple composite reservoir.The study found that with the increase of mobility ratio, the investigation radius reduced gradually, and the decrease rate tended to small. The application confirmed that the method was simple and reliable, and could accurately describe the pressure characteristics of composite reservoir.