In the present paper the design and performance of a waste heat recovery steam generator with variable gas flux is investigated based on second law. A method to calculate temperature and mass flow rate for design is proposed. The effects of the operating parameters and the structure parameters on the average entropy generation number are analyzed. The results show: (i) the average entropy generation number (Ns) decreases with the increase of no-dimensional heat capacities ratio and no-dimensional saturation temperature difference ratio; (ii) compare with the fluctuation of mass flow, fluctuation of temperature has larger effect on the Ns.; (iii) A larger steam parameter is suggested in order to decrease the entropy generation based on reasonable temperature difference and it is appropriate for the number of transfer units of the evaporator and economizer to be 2.5~3.