It is understood that a wall collapsed on the scissor lift bringing the platform down.
Emergency services were called to the site on the A556 Knutsford to Bowdon bypass scheme near the M56.
Cheshure Fire and Rescue Service said: “Firefighters from Cheshire and Greater Manchester were called to a report of a wall collapsing on a scissor lift on a construction site.
“Two site workers had suffered injuries and the crews assisted ambulance paramedics with trauma care. The casualties were taken to hospital.”
A Costain spokesperson said: “An incident occurred where two contractors were injured, one seriously, while working on the A556 Knutsford to Bowdon improvement scheme on Monday evening.
“We are fully cooperating with the relevant authorities as a result of this incident and it is not possible to comment further at this time.”
Costain was awarded the £102m contract late last year with the new link road expected to open in 2016/17.