Athletic defender Talal El Karkouri put the home side ahead on 59th minute when he struck a long range 35 yard shot which took two deflections before rolling over the line.
However, with the pressure high on the Blades shoulders they came back with abundance and Jonathan Stead took his chance with a sweet strike from the edge of the box.
The Addicks almost claimed a winner when Zheng Zhi volleyed wide of goal but the Blades deserved the draw which they held onto.
This match was particularly in the public eye, not only was it a Premier League tie but the game is effectively worth £30,000,000 with the team which stays up earring at least that amount next season.
Charlton were keen to make an impact on their own ground but it was Sheffield United who threw the first punch within two minutes; winger Keith Gillespie crossed the ball over a dangerous low ball which Luton Shelton got on the end of - but was booked for using his hand.
Alan Pardew decided to field an attacking side with Jerome Thomas and Dennis Rommedhal supporting the dangerous Darren Bent, and it paid dividends. Rommedhal was the biggest threat down the right hand side with Matthew Kilgallon unable to keep up with the former Danish international.
Bent, who was only passed fit on his calf injury lost his marker but shot wide from a Darren Ambrose corner before Paddy Kenny tipped over a El Karkouri free-kick.
Neil Warnock, shrewd to realise that there was a problem down left hand side relived early pressure by switching Chris Armstrong to left-back and moving Kilgallon into the centre and Phil Jagielka returning to Midfield.
United then held the foothold in the game, and on 33 minutes Scott Carson tipped Gillespie's header round the post after Armstrong crossed.
And, as the minutes ticked by the tension around the Valley rose, particularly from the home faithful who were desperate for a victory to aid in their push for Premiership survival.
Pardew looked nervous and within ten minutes of the second half saw striker Jimmy Floyd Hasselbaink warming up, but he soon sat down after Athletic took the lead.
Defender Talal El Karkouri was unchallenged as he struck a shot from 35 yards out, it pinged off Michael Tonge and Kilgallon before slipping under Kenny and into the net.
However, even with the lead Charlton were unsettled and ten minutes later, on 69 the Blades took a deserved equaliser. Nick Montgomery slipped the ball through to Jonathan Stead on the right hand side of the edge of the area, and the former Sunderland striker produced a sweet right footed shot past Carson and into the far corner.
Hasselbaink was then dragged on in a latch ditched attempt to claim a win but it was the Blades who almost stole the points, Michael Tonge running in on goal to fire over the bar from edge of the area in stoppage time.