how can I go from the Beijing Capital airport to Huixinxijie Beikou subway station??

You can take the airport shuttle bus line 5 and get off at Xiaoying Station. Then walk westwards for 220 yards to the subway station.

If you want take the metro, you need to transfer several times. Take the airport Expressway line to Sanyuanqiao Station, where you can transfer to line 10 to Huixinxijie Nankou. Then use the line 5 to the destination.