Sims has joined the Royals on a season-long deal.
He said: “I got my chance at Southampton. There were a couple of injuries and I got my break.
"I had a good few games but now is a good time for me to go out on loan and prove myself week in week out and just improve my overall game.
“Rather than playing one game here and there, I'm trying to get a run of games and prove myself."
He added: “I'm delighted. It all happened quickly over the course of the last few days but I'm delighted to be here now and to meet the players and staff and I'm looking forward to the games ahead.
“It's been on the cards a while but in the last few days it's all sped up quite fast which is good because now I'm here and I'm not waiting around.
“I'm quite a direct, fast attacking player, so from what I've seen at the weekend I can definitely incorporate that into the way Reading play.
“I can benefit them and they can benefit me with my progression."