Method and filling installation for filling a hydrogen gas into a vessel (1) having a first port (2) for passing the hydrogen gas and a second port (3) for passing a fluid, wherein the second port (3) is separate from the first port (2). In a first phase a hydraulic fluid is introduced into the vessel (1) through the second port (3) by applying pressure to the hydraulic fluid, such that the hydraulic fluid in the vessel (1) causes a hydrostatic pressure having a pre-determined value. In a second phase the hydrogen gas is introduced into the vessel (1) through the first port (2) by controlling a difference of pressure relative to a current pressure in the vessel (1) for controlling expansion of the hydrogen gas and thereby temperature inside the vessel (1), such that the vessel (1) is filled with the hydrogen gas. To that end the hydraulic fluid is removed from the vessel (1) through the second port (3).