From the Lighthill's exact solution for the incompressible inverse problem it is known that in the inverse design problem，the surface pressure distribution and the free stream speed can not be both prescribed independently. This suggests the existence of a constraint (regularity condition)on the pressure distribution. The same constraint exists at compressible speeds. In this paper，a well-posed inverse design method for transonic airfoil is presented. In the method，the target pressure distribution contains a free prameter that can be adjusted during the computation to satisfy the regularity condition derived in this paper.