In 1630, Jiang Yunlong (c. 1575-?) was sent to Macao to buy European cannon and hire Portuguese mercenaries by the famous late Ming Christian convert Xu Guangqi (1562-1633). Unfortunately, Jiang was accused of corruption and was suspended from his post and prosecuted. More than three hundred well-trained mercenary gunners that Jiang recruited were also sent back to Macao. Due to this incident, efforts to introduce European cannon in the wake of the defeat of the battle of Saerhu (1619) came to an unhappy end. This article investigates Jiang Yunlong's life and his social background to understand why Xu Guangqi chose to consign this important mission to Jiang, and to reexamine the issues surrounding how Western learning penetrated and spread in late Ming society. The results are finally treated as an example how historians can benefit from utilizing newly available massive electronic databases in the so-called ”e-evidential research era.”