Disclosed are a method, device and system for dynamically controlling a gain of a Raman optical fiber amplifier. The method comprises: determining whether a target gain falls within a gain mask range; if the target gain falls within the gain mask range, directly locking a gain to the target gain; and if the target gain falls outside the gain mask range, locking the gain to a corresponding maximum gain in the gain mask range, and gradually increasing the locked gain according to a preset first step length until the target gain is reached or until at least one pump laser reaches a maximum output power. The invention enables an optical fiber amplifier to respond quickly to a change in an input optical signal, ensures gain stability, and ensures that no power overshoot or undershoot occurs in the non-switched optical channels in an optical path. Moreover, the invention minimizes an amount of time required to complete switching between gains.