The two-dimensional, unsteady, viscous, incompressible and asymmetric flow of a micropolar fluid in a uniformly porous channel with expanding or contracting walls is investigated. We assume that the channel walls have different permeability and that the flow is driven by uniformly suction at the lower and upper walls. The corresponding governing equations of motion are reduced to nonlinear coupled ordinary difference equations by a similarity transformation. For the most difficult high Reynolds number case, we construct an asymptotic solution by the method of boundary layer correction. There exist boundary layers near the both walls. Furthermore, this asymptotic results can also be used to validate the numerical method over a range of Reynolds numbers. Finally, the influence of Reynolds number $R$, asymmetric parameter $a$, expansion ratio $α$ and micropolar parameter $n$ on the flow is discussed numerically. The results show that the streamwise and micro-rotation velocity are sensitive to the parameters.