A method, device and system for controlling air supply of fuel cell, and a railway vehicle are disclosed. The method includes: continuously inputting reaction air to a fuel cell based on a preset excess coefficient, wherein the preset excess coefficient is greater than a standard excess coefficient; in a case that a working power of a target object is increased, detecting whether a current excess coefficient is less than or equal to the standard excess coefficient in real time or regularly; and if it is detected that the excess coefficient is less than or equal to the standard excess coefficient, increasing the current excess coefficient to the preset excess coefficient. The disclosure solves the technical problem that the existing fuel cell is relatively slow in response when the electricity consumption demand is increased.