A continuous-flow steam generator includes a combustion chamber having a number of burners for a fossil fuel and a gas-tight containment wall formed from at least approximately vertically disposed evaporator tubes through which a flow passes upwards from below on the feed-water side. A method and a system for starting up the continuous-flow steam generator avoid start-up losses by setting a water level in the evaporator tubes and a ratio of the fuel stream to the feedwater stream in such a way that the water evaporates completely during passage through the evaporator tubes, so that water is no longer present at the evaporator outlet. A start-up system having a setting device for setting the water level in the evaporator and for setting the ratio of the fuel stream to the feed-water stream, is used for this purpose.