Current condition
Demolished / No longer exists
Date completed
Date closed / demolished
Address: Inland Lot No.1, 705 North Point, Hong Kong, as listed by Cinema Treasures. (not found in Google search)
United China Open-Air opened on July 11, 1931, and closed later in the same year coinciding with the closure of the Ming Yuen Gardens.
A condensed discussion on "open-air" cinema:
In 1895 the Lumiere Brothers invented cinematography. Initially, it was possible to show films of up to one minute long. At the start of the 20th century, these 'living photographs' were to be seen in variety halls and theatres, and it was not until 1907 that the first moving cinema was created. It shows photos of a travelling cinema in 1907, a covered Drive-In (not so open-air), outdoor sitting by the water and large LCD screen, and a 1916 photo of one of the world's very first open-air cinemas with rows of portable chairs and a large screen all surrounded by greenery. I believe the last example best describes how the movie-goers experienced at Ming Yuen Gardens.
Cinema Treasures describes the cinema as part of the Ming Yuen Gardens in Tsat Tsz Mui on Hong Kong Island. It shows a photo and marker on Electric Road just east of Watson Road. However, Ming Yuen Gardens and Tsat Tsz Mui Road are about 400 metres to the east.
https://gwulo.com/node/20041#16/22.2886/114.1972/Map_by_ESRI-Markers/100 . The location marker on this map has been placed in Ming Yuen Gardens close to Tsat Tsz Mui Road, the spot I consider best reflects the discussion by Cinema Treasures.
Sources: Cinema Treaures: http://cinematreasures.org/theaters/31208