The year after the Japanese army touched off the Mukden Incident in September 1931, the NPM began its retreat to the south. All pieces were boxed and coded using a character from the Thousand Character Classic to identify which building they had been kept in at the Forbidden City.
1931 年，日軍發動「918 事變」，隔年，故宮文物開始南遷，文物按其存放的殿室，以《千字文》的文字為代號，分別裝箱。