The invention discloses a steam source switching method for a thermal power generating unit during FCB. The steam source switching method comprises the following steps that when the thermal power generating unit runs independently under severe natural weather or peak electricity consumption, an auxiliary steam supply small electromechanical door is controlled to be opened in advance; if the auxiliary steam supply small electromechanical door is not opened in advance, during the FCB action, the auxiliary steam supply small electromechanical door is controlled to be opened; during the FCB action, if the auxiliary steam supply small electromechanical door is not opened, overshoot control is carried out so as enable a small machine high-pressure regulating valve to be started to enable a coldre-supply steam source to be connected into a small steam turbine; during the FCB action, a cold re-supply auxiliary steam adjusting valve is opened to a preset opening degree so as to maintain the pressure of an auxiliary steam master tube to be stable; and during the FCB action, recirculation of steam-operated water feeding pumps is started so as to be switched to operation of the single steam-operated water feeding pump, and another steam-operated water feeding pump can be kept to rotate for later use.