He participated in the pro-democracy primary of the March 2018 Kowloon West by-election after incumbent Yau Wai-ching of Youngspiration was ousted from the Legislative Council over the oath-taking controversy. His decision of running sparked intra-party conflicts. His candidacy was initially challenged by party's young member Kalvin Ho who latter withdrew from the race. Former vice-chairwoman Rosanda Mok and all ADPL's Yau Tsim Mong and Kowloon City District Councillors quit the party in the midst of the intra-party primary, in protest against Fung's candidacy. Fung eventually became the party's sole representative in the pro-democracy primary. He was defeated in the telephone polls and generic ballots in the primary to Yiu Chung-yim, the legislator who was also disqualified over the oath-taking controversy. According to the primary mechanism, Fung was a backup candidate in case Yiu was barred from running in the election. However, announced that he was under pressure to withdraw as a backup candidacy as agreed in the primary if Yiu was disqualified by the Electoral Affair Commission. "Someone from the progressive democracy bloc told me that if I ran in the poll, they would definitely send someone as well [to challenge me],” Fung said. “I hope my withdrawal will allow the hatred to dissipate."