A new theory for polymer gels is developed on the basis of the single-chain mean-field (SCMF) theory. This theory takes the mesh chain or the star polymer as the characteristic unit of polymer gels, and it allows the study of chain conformation properties for gels in their different states.The preparation condition effect is conveniently described by the current theory, and the gels with complex chain structures can also be studied since the SCMF theory take realistic chain conformations into consideration. In this work, the theory is compared with the widely used Flory-type gel theory and with experimental observations. It is shown that the star polymer is a more natural basic unit of the gel structure. The current theory is found to provide a better description of the gels than the Flory-type theory as well as better agreement with gel swelling experimental data for poly(ethylene oxide) gels.