Professional discourse analysis, particularly of advertisement genre and business genre, has begun to garner nationwide attention since the introduction of genre theory into China in the late 1990s. Based on the previous researches on promotional genre, this paper investigates the differences between Chinese and English sales promotion letters at both text-internal and text-external level. Text-internally, the study focuses on the generic pattern and interactional metalanguage of the sales promotion letters, while, text-externally, it examines the relationship between pragmatic strategy and generic variations. It is found that great variations exist in some particular generic moves and metalanguage; in a socio-pragmatic space different politeness strategies, against various social cultural backdrops, are exploited to fulfill shared communicative purposes in these letters. This study might help genre analysts better understand text-external factors such as politeness strategy that contribute to generic variations. Meanwhile, the results of the study bear pedagogical implications for ESP/ EAP writing courses.