In the March 2018 Legislative Council Kowloon West by-election triggered by the disqualification of Youngspiration's Yau Wai-ching over the oath-taking controversy, Cheng defeated another DAB member Chris Ip Ngo-tung in an intra-party selection to represent the party in the election. He made an upset in Kowloon West by narrowly defeating independent democrat Yiu Chung-yim, making it the first time the pro-Beijing camp received greater vote share than the pro-democrats in a geographical constituency since 2000 and the first time a pro-Beijing candidate won in a geographical constituency by-election since 1992. He lost his seat on the District Council in the 2019 District Council elections in Hong Kong.