Goals from Neil Mellor and Tamas Priskin looked like giving the visitors a resounding victory.
But ex-Preston star Ainsworth, on as a substitute, pulled a goal back with a brilliant looping shot and then crossed for Dexter Blackstock to head in a dramatic equaliser.
Preston went ahead on 37 minutes when Paul McKenna's right-wing corner was headed on by Youl Mawene and Mellor volleyed home from five yards.
It was a deserved breakthrough for North End, who had twice been denied by Rangers keeper Lee Camp early on.
Camp rushed from his line to block Mellor's close-range shot and was soon called into action again, this time to save from Simon Whaley after the winger had ghosted in to collect Chris Brown's clever pass.
QPR seemed to be finding a way back into the match when a Blackstock shot brought a good save from keeper Andy Lonergan, but Mellor then put Preston ahead.
Mellor, who seemed to be struggling with a hamstring strain, was replaced at half-time by on-loan Watford striker Priskin.
Preston continued to dominate and appealed in vain for penalty when Priskin went down in the area under a challenge from Fitz Hall.
There was no escape for Rangers when Camp failed to gather Chris Sedgwick's low cross and Priskin bundled in Preston's second goal.
Rangers did not look like recovering and Preston twice came close to going further ahead.
Priskin's header from McKenna's corner was pushed away by Camp, before Darren Carter's free-kick was headed wide by the unmarked Sean St Ledger.
But after Ainsworth netted spectacularly from 20 yards, the veteran winger then set up Blackstock to complete an amazing turnaround.