[Objectives]  Fall armyworm (FAW; Spodoptera frugiperda) caterpillars were first detected in China a corn field in Baozang Township, Jiangcheng County, Yunnan Province on December 26, 2018. This study aims to clarify the timing of this invasion and the origin of these first fall armyworm colonists. [Methods]  An effective accumulated temperature model was used to estimate the developmental period and time of migration of the first fall armyworm arrivals in China. Their potential origin was identified using a three-dimensional trajectory analytical approach, and the effect of weather conditions on their migration was also analyzed. [Results]  The first fall armyworms to arrive in China probably migrated from the border with Myanmar and Thailand to Jiangcheng between 21 and 23 November, 2018. The weak southeasterly winds that prevailed in southern Yunnan in November and December 2018 were unsuitable for windborne migration, but a continuous southwest airflow between 19 and 22 November was conducive to migration from the border with Myanmar and Thailand to Jiangcheng. [Conclusion]  The estimated invasion time and origin of the first fall armyworms detected in China can inform integrated pest management strategies for this emerging pest.