Abstract: A thermo-economic model for 100 MW trough solar thermal power plant based on Rankine cycle with molten salt heat storage is established to study the optimal combination of the design parameters such as design direct normal irradiance（DNI）,solar multiple and thermal storage hours under different solar radiation conditions. The effects of these parameters on the annual power generation,annual power generation efficiency and average power cost are identified,and then a novel method for determining the design DNI for trough system is proposed. The results show that the optimal design DNI for minimal levelized cost of energy depends on both the annual irradiation level and the distribution of solar irradiance, which can be determined by the specific percentage of the annual radiation above the design DNI in the total annual radiation. Sensitivity analysis by changing the main financial parameters shows that only solar field cost leads to slight change in the optimal design DNI and the corresponding specific percentage.