There is a downward entrance in the middle of the platform of Dongsishitiao Station of Beijing Subway, which leads to the lower floor. Does anyone know what it is?

That's the long-term Line 3 station reserved during the construction of 1984 subway phase II project (Beijing Station-Fuxingmen-Changchun Street). At that time, the planning trend of Line 3 was: from Xiangshan along Haidian section of Line 4 to Ping 'anli, then along Ping 'an Street to the east, through Dongsishitou, and through Maliang River to Wangjing. However, the second edition plan of Beijing Metro revised by 1999, Line 3 has not been implemented, and the underground second floor station has not been used as the office space of the subway police station.

The stairs you see are the transfer to the next stop. But only the platform, no track and no circuit.

Now the newly planned Line 3 doesn't go that way. According to the latest planning, the route of the station is 16, from Huilongguan to Ping 'anli, and finally to the East Fifth Ring Road.