unflappable
adjective uk/ʌnˈflæp.ə.bəl/ us/ʌnˈflæp.ə.bəl/
镇定自若的，沉着冷静的
not likely to get worried, nervous, or angry even in difficult situations
She's totally unflappable - you have to be when working in such a high-pressure environment. 她非常沉着冷静——在压力如此之大的环境下工作必须这样。
Synonym: imperturbable