The optimisation of ultrasound extraction of semen astragali complanati flavonoids was studied by measuring characteristic absorbance at 266 nm as the response and using response surface methodology (four-variable, three-level Box–Behnken design, BBD) in this article. The optimal conditions were obtained as 52 °C, 34 min, 26:1 (mL:g) and 100 mesh (0.120–0.150 mm) for extraction temperature, extraction time, solvent-to-sample ratio as well as particle size, respectively. Under these conditions, validation experiments were carried out and the experimental value of A266 was 0.9907 ± 0.032 (n = 3), which corresponded to an experimental extraction yield of 7.08%. Compounds in the extracts obtained with the optimum extraction conditions were identified by high-performance liquid chromatography coupled with diode array and mass spectrometry detectors (HPLC–DAD–MS). Among the 14 compounds that were tentatively identified in the extracts according to their ultraviolet-visible spectroscopy, mass spectrometry and related literature reports, four were reported for the first time.