A method for operating a waste heat steam generator including an evaporator, an economizer having a number of economizer heating surfaces, and a bypass line connected on the flow medium side in parallel with a number of economizer heating surfaces is provided. The method makes possible higher operational safety and reliability in the control of the waste heat steam generator. For this purpose, a parameter that is characteristic of the thermal energy fed to the waste heat steam generator is used to control or regulate the flow rate of the by-pass line.