Allen could not reach a final in the first half of the season, he lost in all six semi-finals which he reached. In International Championship and Shanghai Masters Allen lost against Shaun Murphy, in English open against Mark Selby and in Champion of champions lost to Judd Trump. In the UK Championship, he suffered a whitewash from Stephen Maguire. Allen's attempt to regain his Scottish open title in 2018 ended in the decider of the semi-final match against Jack Lisowski. At the World Championship Allen equalled the record of making 5 centuries in a first round match against qualifier Jamie Clarke; however, he went on to lose the match against the Welsh number 89 - the lowest ranked player in the tournament- 8-10. He ended the season where he finished the last one, ranked number 5.