Ionic liquid-based salt-induced liquid-liquid extraction was developed for the first time and applied to the extraction of four active constituents, including polydatin, resveratrol, emodin, and physcion in Polygonum cuspidatum (P. cuspidatum). In this study, ionic liquid was used as extraction solvent. The dried P. cuspidatum samples purchased from the pharmacy were triturated and passed through a 120-mesh sieve. The obtained sample powders were dried to constant weight at 55 °C, and then mixed with extraction solvent. The extraction was carried out with the aid of ultrasound. Three phases, including ionic liquid-rich, salt-rich and solid sample phases were formed in the presence of salt. The target analytes were enriched in ionic liquid phase and then determined by high performance liquid chromatography. The experimental parameters, such as type and volume of ionic liquid, type and amount of salt, pH value of extraction medium, ultrasound power, ultrasound time and centrifugal condition, were optimized. The calibration curves showed good linear relationship (r > 0.9994). The limits of detection and quantification were in the range of 2.8–29.5 and 9.4–98.3 ng mL−1, respectively. The spiked recoveries were between 92.16% and 105.41%. Compared with hot reflux extraction and ultrasound-assisted extraction, the proposed method requires less extraction solvent and time. The present method can be applied to the determination of polyphenols and anthraquinones in P. cuspidatum.