Field experiments were conducted using two sweet potato cultivars (Jishu 21, a drought-tolerant cultivar, and Jizishu 1, a drought-sensitive cultivar) with four water treatments to investigate the effects of drought treatments at different growth stages on growth and the activity of osmotic adjustment in sweet potato [Ipomoea batatas (L.) Lam.], including well-watered treatment during the whole growth period (WW, control), drought stress during the establishment stage (DS1), drought stress during the storage root initial stage (DS2), and drought stress during the storage root bulking stage (DS3). Drought stress resulted in significant decrease of fresh weight of storage roots in sweet potato. Compared with drought stress in different periods, drought stress during the establishment stage (DS1) decreased the fresh weight most. Compared between cultivars, drought-sensitive cultivar decreased the fresh weight most. The average in three years, compared with the control, the fresh weight of drought-tolerant cultivar (Jishu 21) and drought-sensitive cultivar (Jizishu 1) decreased 28.59% and 38.77% in DS1 treatment, respectively, while 25.20% and 33.50% in DS2 treatment, respectively and 14.55% and 19.56% in DS3 treatment, respectively. Drought stress resulted in significant decrease of biomass of storage roots in sweet potato. One hundred days after planting, compared with the control, the biomass of aboveground part of Jishu 21 in DS1, DS2, and DS3 decreased 32.68%, 20.79%, and 11.72%, respectively, while Jizishu 1 decreased 46.45%, 31.89%, and 18.43%, respectively. The biomass of underground part of Jishu 21 in DS1, DS2, and DS3 decreased 37.69%, 25.86%, and 10.67%, respectively, while Jizishu 1 decreased 54.34%, 33.48%, and 14.20%, respectively. Under drought stress, the relative water content of functional leaves decreased, and the content of soluble sugar, soluble protein, free amino acid and proline in functional leaves, fibrous roots and storage roots increased. The earlier the application of drought stress, the greater the decrease or increase. The effects of drought stress applied at early stages on osmotic adjustment could not be effectively recovered after re-watering, while the osmotic adjustment could be recovered to the control level after re-watering when drought stress was applied at later stage.