Water shortage leads farmers to use sewages for irrigation. Sewages contain a large amount of laundry detergent. In this study the impact of irrigation by contaminated water on shoot biomass and seed germination of foxtail millet (Setaria italica) was investigated. The research was conducted as laboratory and pot experiments. Iso-potentials (- 0.042, - 0.077, and - 0.415 MPa) of polyethylene glycol (PEG, water deficit treatment) and laundry detergent (contamination treatment) made the laboratory experiment treatments. The pot experiment included contamination factor (0, 0.1, 1, and 10 g L of laundry detergent) and deficit irrigation factor (irrigation interval of 1, 2, and 3 days). Results of this study showed that at the iso-potential, laundry detergent had more negative effect on seed germination traits when compared with PEG. There was no germination at - 0.415 MPa of laundry detergent. Both drought and contamination reduced dry forage yield, plant height, leaf number, leaf area, leaf dry and fresh weight, stem dry, and fresh weight. Detergent concentration of 10 g L with irrigation interval of 3 days had a forage yield reduction of 63% compared to control (laundry detergent concentration of 0 g L with irrigation interval of 1 day). Detergent concentration of 10 g L with irrigation interval of 1 day had a sodium increase of 1847% compared to control. Based on the results of this study, it is recommended not to irrigate foxtail millet farm by contaminated water with laundry detergent higher than 1 g L.