After trailing 5-4 overnight, Cao got himself 9-6 ahead in the second session.
Allen looked to have got back in contention in the 16th frame, but he saw the white roll in after he had potted the black to give Cao a surprise win.
"It's very disappointing," he said. "It turns a decent season into a mediocre one."
The victory has ensured that Cao will at least double his previous career winnings.