Ren first appeared internationally in March 2014 at the Junior World Championships in Erzurum . There he won the bronze medal over 1500 m and the gold medal with the relay. In the following year he won the silver medal at the Junior World Championships in Osaka over 1000 m and with the relay. He made his debut in the Short Track World Cup in Nagoya in December 2015 and finished 26th over 1500 m and 21st place over 1000 m. At the following World Cup in Shanghai , he won his first World Cup victory over 1500 m. At the Junior World Championships in Sofia in 2016 , he won the silver medal in the 1500 m and gold medal in the all-around, over 1000 m, in the 1500 m superfinal and with the relay. In the 2016/17 season he won the World Cup in Salt Lake City and Shanghai with the relay. At the 2017 World Championships in Rotterdam , he won the silver medal with the relay. In February 2017 he won the gold medal with the relay at the Winter Asian Games in Sapporo . In the 2017/18 World Cup season he finished second in Budapest and third in the relay in Dordrecht . His best individual result of the season was fourth in Seoul over 500 m. At the season highlight, the 2018 Winter Olympics in Pyeongchang , he won the silver medal with the 5000 m relay. He finished sixth in the 500 m run. In March 2018, he won the silver medal over 500 m at the World Championships in Montreal .
In the 2018/19 season, Ren achieved third place over 1000 m in Calgary and second place in Salt Lake City over 1000 m, tenth place in the World Cup over 500 m and fourth place in the World Cup over 1000 m. He also won in Calgary with the mixed relay and won second place in Salt Lake City with the relay. At the 2019 World Championships in Sofia , he won the bronze medal over 500 m and the silver medal with the relay. In the all-around competition he took eighth place there. In the following season he got three wins, two of them with the relay and two third places and thus finished sixth in the World Cup over 1500 m.