Gamified learning tools refer to the websites, software or mobile applications that use game design elements for educational purposes. Though theoretically promising in engaging leaners, gamified learning tools were not widely accepted in practice. This paper seeks to better understand what encourages and constrains teachers' use of gamified learning tools in secondary schools in China in order to better realise the potential of gamification in formal learning contexts. The research comprises a survey and an interview that involves 347 and 14 secondary school teachers respectively, who were recruited with the use of convenience sampling method. Using the thematic analysis approach, the author obtained 16 themes that influence teachers' acceptance intention to gamification, followed by a list of sub-themes that furtherly explain the themes. Research limitations, implications and future research lines were addressed.