在某人的业余时间英文：In one's spare time。
业余时间是指：闲暇时间，亦称业余时间、“自由时间”。指人们除劳动外用于消费产品和自由活动的时间。美国经济学家凡勃伦于1899年创用。他认为闲暇是指不生产的消费时间。
人们在闲暇时间中进行生活消费，参与社会活动和娱乐休息，这是从事劳动后身心调剂的过程，与劳动力再生产及必要劳动的补偿相联系。对社会而言，闲暇可以创造产生科学、艺术等；对个人而言，闲暇可使人的个性得到全面发展，进行文化享受和创造活动，得到娱乐和休息。
双语例句：
1、Chen Chengpei, an office worker in Xiamen City loves reading on his mobile phone app in his spare time.
厦门市上班族陈成培喜欢在业余时间用手机软件阅读。
2、In 2001, the younger Lei found a job in Shanghai and began to read books related to ancient poems at bookstores or libraries in his spare time, since he didn't make much money.
2001年，小雷在上海找到了一份工作，由于收入不高，他开始利用业余时间在书店或图书馆阅读古诗的相关书籍。
3、He's studying music in his spare time.
他在空闲时间学音乐。
4、Besides working as a doctor, he also writes novels in his spare time.
除了当医生之外，他在业余时间还写小说。
5、He would rather stay alone than gossip with his co-workers in his spare time.
他宁愿一个人呆着，也不愿在业余时间和同事们闲聊。
6、With the help of a dictionary, he read English magazines and books in his spare time.
在字典的帮助下，他在业余时间读英语杂志和书籍。