  In this paper, we propose a method of solving the viscous hydrodynamics order
by order in a derivative expansion. In such a method, the zero-order solution
is just one of the ideal hydrodynamics. All the other higher order corrections
satisfy the same first-order partial differential equations but with different
inhomogeneous terms. We take the Bjorken flow as an example to test the
validity of our method and present how to deal with the problems about the
initial condition and perturbation evolution in our formalism.
