A method and system for treating, by filtration, haemorrhagic fluid contained in a container with a view to subsequent autotransfusion, including at least one step of concentrating by filtration said haemorrhagic fluid in order to increase the concentration of red blood cells in the haemorrhagic fluid to reach a target haematocrit level while at the same time removing from the haemorrhagic fluid a filtrate comprising compounds not desired for autotransfusion, further including: a preliminary step of measuring the haematocrit level of the haemorrhagic fluid; and a dilution step consisting in adding to the volume of haemorrhagic fluid to be treated a determined volume of dilution fluid, the determined volume of dilution fluid being calculated as a function of the measured haematocrit level of the haemorrhagic fluid and the target haematocrit level.