The invention provides a volume variable assembly used in a nuclear power station, comprising a volume changing unit arranged in a fluid container. The volume variable unit is suitable for expansion or contraction in the fluid container based on trigger signals, and the variance is controllable, thus the volume variable unit is used for changing the fluid level in the container and the pressure in the container. The invention further provides a method for changing a gravity safety injection pressure head in the nuclear power station safety injection system, comprising the following steps: arranging the volume variable unit at the bottom of the fluid container; allowing the volume changing unit to expand or contract in the fluid container based on the trigger signals to change the fluid level in the fluid container, so as to change the safety injection pressure head. According to the invention, nuclear power station safety accident can be alleviated, and the safety of the nuclear power station is raised.