Mr Miliband, who previously 'absolutely' supported the high speed rail link, now thinks the project will 'disrupt' lives and hit the countryside.
He said: "It's important to try and stop these proposals by making the case for an alternative route. The more that I hear about this proposed HS2 route, the worse it sounds.
"It will evict families from their homes, disrupt people's lives, and have a significant impact on parts of our beautiful countryside.
"I will work with concerned residents, councillors, the Mayor of Doncaster, and other MPs.
"We need to do all we can to persuade the government of the case for it, based on the economics as well as the effects on my constituents.
"I am also determined to get answers from HS2 to the many real questions my constituents have, and will continue to meet residents, HS2 and the government in the weeks ahead.
"If the government continues to insist on riding roughshod over my constituents, I will also work to secure the best possible deal for them."
But in the run-up to last year's general election, the then-Labour leader re-affirmed his personal and party's commitment to the line, which will link London with Birmingham and split in two to Manchester and Leeds.
The line will run for 45 miles through Staffordshire countryside, cutting diagonally from the south of Lichfield to north of Stafford.
Despite opposition to the scheme in Staffordshire, Cheshire, Warwickshire, Buckinghamshire, and Northamptonshire, Mr Miliband said last year: "We support HS2, absolutely. We are not backing off it, we are not changing our position. We are supporters of HS2 and that's clear."
Mr Miliband's change of heart comes after HS2 Ltd, the government-owned group running the project, altered the route to run into Sheffield city centre.
The new route could see the demolition of more than 200 homes on the Shimmer Estate in Mexborough in Mr Miliband's Doncaster North constituency.