**4. Conclusions**

In this work, the UAE was proved to be a sample, rapid, and effective procedure for the extraction of TF from *P. cretica*, and the process parameters optimized by RSM were as follows: Ethanol concentration 56.74%, extraction time 45.94 min, extraction temperature 74.27 ◦C, and liquid/solid ratio 33.69 mL/g. Under optimized UAE conditions, the TFY was 4.71 ± 0.04%, which was higher than that obtained by HRE. Moreover, HPLC analysis indicated that luteolin-7-*O*-glucoside, rutin, quercitrin, luteolin, and apigenin were the predominant flavonoids in *P. cretica*. It was also found that UAE is more suitable than HRE for the extraction of these compounds. In addition, the *P. cretica* extract obtained through the optimized UAE method exhibited good DPPH•, ABTS+•, and NO• scavenging activities. The results of the present wok should contribute to the further and deeper investigation of *P. cretica*, which might be used as a novel source of natural antioxidants.

**Author Contributions:** Conceptualization, W.H.; methodology, W.H. and M.H.; software, A.W., K.H., and X.S.; validation, W.H. and Z.X; formal analysis, D.C.; investigation, M.H. and R.L.; resources, W.H.; data curation, M.H. and J.S.; writing—original draft preparation, M.H.; writing—review and editing, W.H.; supervision, Y.S.; project administration, W.H.; funding acquisition, W.H.

**Funding:** This research was funded by the "Thirteenth Five-Year Plan" for the National Key Research and Development Program (No. 2016YFD0400903) and National Natural Science Foundation of China (No. 31471923).

**Conflicts of Interest:** The authors declare no conflict of interest.
