A new method called solid-phase microextraction has been developed to identify bioactive compounds in medicinal plants. The method avoids the use of large amounts of organic solvents and is environmentally friendly. Polyvinylidene fluoride fibers coated with polydopamine and polyethyleneimine were used as a solid part of the extractors. The fibers were placed in a sample solution of medicinal plants, stirred and then removed. The method was optimized by varying the temperature, ion strength, and pH. Nine bioactive components were obtained from Plantago depressa extract. The method had high precision, accuracy, and stability, and was confirmed through surface plasmon resonance biosensor. This new method is a powerful tool for locating bioactive compounds in medicinal plants.