  A new perturbation theory is proposed for studying finite-size effects near
critical point of the $\phi^4$ model with a one-component order parameter. The
new approach is based on the techniques of generating functional and functional
derivative with respect to external source field and can be used for
temperatures both above and below the critical point of the bulk system. It is
shown that this approach is much simpler comparing with available perturbation
theories. Particularly, this new method avoids renormalization in calculating
many physical quantities such as correlation functions etc..
