This study introduced an app, called Smart UEnglish, for helping EFL conversation practices in authentic contexts. These conversation practices were categorized into 'designed talk' and 'free talk', based on the content of an English textbook and authentic ambient environment that includes such things as transportation, weather and scenic descriptions, respectively. The mechanisms in Smart UEnglish were designed in consideration of flexible, sustainable and adaptive conversations to improve English-speaking and lexical resources. Statistical analysis results show that the learning achievement of the experimental group is significantly higher than that of the control group. Moreover, both 'designed talk' and 'free talk' have significant positive correlations with learning achievement. Further analysis with stepwise multiple regression found that 'designed talk' plays the most important role and can significantly influence learning achievement. This is because 'designed talk' can motivate them to practice what they learned in class during their daily conversations and consolidate the understanding and applying of English vocabularies and sentence patterns through 'designed talk'. According to interview records, participants also perceived that using Smart UEnglish for conversation practice in authentic contexts can stimulate their interests and ideas to think and practice more useful English conversation. Hence, Smart UEnglish can effectively help English-speaking and conversation in authentic contexts anytime and anywhere.