A new kind of fiber grating - active fiber gratings is proposed to avoide the energy loss in optical pulse switching process of nonlinear fiber gratings. This paper studies the characteristics of switching dynamics in erbium-doped nonlinear fiber gratings under cw light. The results show that under cw light, the switching of the proposed fiber grating is a phase-transition-like process. As a cw light switching device, the active fiber grating is not as good as expected, but it is advatageous as a pulses generator.