This paper presents a theoretical analysis of a basin type still with internal and external reflectors. We propose a geometrical method to calculate the solar radiation reflected by the internal and the external reflectors and then absorbed on the basin liner. We also performed numerical analysis of heat and mass transfer in the still, and found that the internal and the external reflectors can remarkably increase the distillate productivity throughout the year except for the summer season, and the increase in the daily amounts of distillate by adding the internal and the external reflectors to the single-slope basin type still for the entire year would be averaged as 48%.