Yuan started to play snooker at the age of 10 and played as a wildcard in many professional tournaments between ages of 13 and 16. Yuan first drew attention in 2016 when, at the age of 15, he defeated world number 13 Martin Gould in a 5–0 whitewash in the opening round of the 2016 China Open before losing 2–5 to Graeme Dott.. Remarkably, when he again faced Martin Gould in the 2016 Shanghai Masters in October 2016, he produced the same result, a 5-0 win.
In 2015, Yuan entered the 2015 ACBS Asian Under-21 Snooker Championship where he reached the final, but lost eventually to Akani Songsermsawad. Two years later, Yuan once again made it to the final, where he defeated Fan Zhengyi 6–2 to win the Asian Under-21 Championship; as a result, he was given a two-year card on the professional World Snooker Tour for the 2017/2018 and 2018/2019 seasons. Yuan began the 2017/2018 as the youngest player on the professional tour.
Yuan's first match as a professional was a narrow loss to Oliver Lines in the 2017 Riga Masters qualifiers on 31 May 2017, two days after his 17th birthday. His first win was a 4–2 defeat of Allan Taylor in the European Masters qualifiers.. Yuan's best wins were 4-1 against Joe Perry in the English Open (which included two centuries), and 5-4 against Shaun Murphy in the Shanghai Masters. In the World Championship, he beat Fergal O'Brien 10-5, before losing 10-9 to Matthew Stevens, despite leading 9-7 and missing several chances to win. In total Yuan managed only 8 wins in his first season, and ended the season ranked 92.
With wins over Ricky Walden, Alexander Ursenbacher, Ding Junhui and Mark King, Yuan reached his first quarter-final at the China Championship, before losing to World No.1 Mark Selby. For the next few months, he progressed steadily, beating the likes of Mark Allen and Kyren Wilson. In the German Masters, he beat 4-time World Champion John Higgins 5-4 on the final black, somewhat controversially as the cue-ball appeared to touch the rest (a foul stroke) after the players had shaken hands.
Yuan's solid results in the first half of the 2018/19 season lifted him into the top-32 on the 1-year rankings, thus qualifying for the World Grand Prix in Cheltenham. He beat World Champion Mark Williams 4-3 and Stephen Maguire 4-2 to reach his second quarter-final, losing to flatmate Xiao Guodong 5-4. He attracted praise from commentators Jimmy White, Ronnie O'Sullivan and Stephen Hendry, who described him as 'the best 18-year old since Ronnie O'Sullivan'.
In the International Championship Yuan suffered an agonising loss on the final black to Shaun Murphy 6-5, despite making 8 breaks over 50. This contributed to a loss of form, reaching the last 16 only once, in the Northern Ireland Open.
At the start of the 2018/2019 season, Yuan Sijun moved to Darlington and is based at the Q House Academy, managed by Chusak Phetmalaikul.