By constructing the coupled-mode equations, the mode-coupling characteristics between the passive and active cores within a kind of composite structural fiber (CSF) are studied theoretically. By using the steady-state rate equations and the heat conductive equations, the gain characteristics and the temperature distribution of all-fiber laser amplifiers based on CSF are calculated and analyzed numerically. The results show that the mode-coupling characteristics depend on the coupled modes, the radius and the separation of the two cores of the CSF. Compared with the end-pumped fiber laser amplifiers, the amplifiers based on CSF have a slow pump light absorption and conversion, and thus, have low temperature distribution. This kind of new coupling-pump technology by using CSF is of apparent advantages. It can be used to solve the problem of high temperature of end-pumped fiber amplifiers, and thereby provides a new approach to design higher power fiber lasers and fiber amplifiers.