Walker, ranked 80th in the world, toppled world champion Judd Trump earlier in the tournament but was unable to lay a glove on Allen, who put together runs of 54, 51 and 80 to win a scrappy affair.
The 37-minute third frame, won by Allen, gave the Northern Irishman the momentum to take a four-frame advantage to the mini-interval, and while a typically dogged Walker took the fifth frame, it merely delayed the inevitable.
And Allen sealed his spot in the semi-final at the second time of asking to set up a showdown with Mark Selby.
“The only frame I missed balls was the third really – the rest of it was alright,” said Allen after the match.
[The semi-final against Mark Selby] is going to be a tough match. You know what Mark brings – he scores well and his all-round game is very good and he is good under pressure so you have to play well.
Earlier in the evening session, Tom Ford, who produced a maximum in his deciding frame against Shaun Murphy at the last-16 stage, whitewashed Tian Pengfei 5-0.
Ford, ranked 25th in the world, produced breaks of 62, 52 and 128 on his was to a first semi-final of the season.
Ford will face Dave Gilbert in the semi-final at 20:00 LIVE on Eurosport and Eurosport Player after Allen and Selby duel it out from 14:00.