Reading went ahead midway through the second half when Garath McCleary fired home a low drive from 25 yards but the visitors drew level when Barton clipped in a free-kick near the end.
QPR, after three matches without a victory, had got back on track with a 2-1 home win over Derby in their previous fixture and they impressed in the opening stages at the Madejski Stadium.
Junior Hoilett struck the bar with a thunderous 30-yard drive and Charlie Austin was twice denied by sprawling saves from Reading goalkeeper Alex McCarthy.
The Royals, thumped 5-2 by Sheffield Wednesday last time out, appeared to be on to another hiding but they gradually gained a foothold.
McCleary blazed over after cleverly creating space for himself and Pavel Pogrebnyak, after a goalmouth scramble, jabbed the ball just wide from a McCleary cross.
Pogrebnyak nodded over a Chris Gunter cross as Reading piled on the pressure and QPR keeper Robert Green had to make a fine save from Chris Baird's glancing header from a Danny Guthrie free-kick.
Back came QPR approaching half-time, with Barton pulling most of the strings in midfield.
McCarthy was again in action to deny Austin, keeping out his firm header from a Danny Simpson cross, then McCarthy saved superbly from a low drive from the industrious Hoilett.
Despite the lack of goals, the entertainment continued in the second half. QPR remained mostly in control but, as they pressed forward, were thwarted by strong last-ditch blocks by home defenders Kaspars Gorkss and Stephen Kelly. Barton also blazed over from long distance.
QPR, boasting nine previous clean sheets this season, again appeared untroubled at the back but were undone in the 62nd minute, when Reading went in front.
Baird released McCleary and there seemed little danger but the winger cut in from the left and unleashed a low shot that struck the foot of an upright and rolled past Green.
Reading went in search of a second goal and almost got it when Jordan Obita managed to escape the clutches of the QPR defence. But with only Green to beat, he drove his effort against the keeper's legs.
It would prove costly for Reading, with the visitors equalising in the 78th minute. Gorkss clumsily fouled Austin on the edge of the home area and Barton stepped up to take the free kick.
His well-struck effort was on target and, with Guthrie and Gunter turning their backs on it and creating a gap in the defensive wall, a route to goal was cleared. McCarthy had little chance as the ball curled into his left-hand corner of the net to secure QPR the draw.