For the general case (either paraxial approximation or non-paraxial situation) and the case of discrete sample, we show that any given linear transform can be achieved by dual phase-amplitude mask optical system. Using feed-back iteration approach, the specific formulae to design the phase-amplitude distributions of dual masks are obtained. Adopting four dimension al Walsh transfom in three orders as examples, we have given the phase-amplitude distributions for masks and evaluated the approiach degree. These results axe relatively satisfactory. It shows the present proposal is reasonable and useful.