Dean Morgan capped off a fine break by Craig Westcarr to put Chesterfield ahead in the basement battle after only four minutes but Anthony McNamee, on loan from MK Dons, levelled with a fine volley on his Wycombe debut.
Morgan grabbed his and Chesterfield’s second after the break to temporarily lift the visitors off the bottom of the table only for Fulham loanee Marcello Trotta to level and then Beavon to steal all three points.
The visitors took the lead after Westcarr broke from a Wycombe corner and left Morgan with the simplest of finishes.
Wycombe saw Scott Donnelly stretchered off but equalised when McNamee volleyed in on 31 minutes after Leon Johnson and Dave Winfield had both been denied.
Morgan grabbed his second on 54 minutes after a long goal kick split the Wycombe defence, but Trotta grabbed his second in as many games as the clash approached the closing stages before Beavon got his head on a long free-kick to win it in injury time.