A soldier told him of an escape route back to China and Wu Rui escaped to Longzhou.
In early 1940, troops of the Imperial Japanese Army (IJA) moved to seize Longzhou in south Guangxi, where the eastern branch of the railroad from Hanoi reaches the border, and also tried to move west to cut the rail line to Kunming.
From this point on, he performed field studies of several Tai languages (including the Zhuang people's Longzhou and Wuming dialects), while at the same time conducting deep investigations into Old Chinese and Tibetan.