Abstract: There are many tube bundles subjected to two-phase cross flow in heat exchangers, such as steam generators. For calculating the vibration response of tube bundles caused by two-phase flow turbulence, the power spectrum density (PSD) of buffeting forces should be obtained firstly. Since there is no generally accepted method for normalizing the buffeting forces, the acknowledged upper bound of buffeting forces caused by two-phase flow is absent. By modifying the definition of mixture velocity used in the de Langre’s nondimensional normalizing procedure, a new set of upper bound of buffeting forces caused by two-phase flows was obtained. The new upper bound was compared with the one based on single-phase flow and de Langre’s upper bound. Through a sample of steam generator tube subjected nonuniform two-phase cross flow, the random vibration responses were calculated with the three types of upper bounds. The results show that the upper bound based on single-phase flow is not conservative as the input for calculating the vibration response excited by two-phase cross flow, and the new upper bound reduces the excessive conservatism of de Langre’s bound in the premise of ensuring the safety.