Police are on the scene and drivers have been urged to take another route.
The road is closed northbound at Lowries Den and southbound at Damhead Junction.
Police were called at 7:45 following reports of a crash involving a car and a lorry.
A police spokesperson said: “Police in Midlothian are currently in attendance on the A702, near Boghall farm, following a serious collision involving a car and an HGV which took place at around 7.45am on Tuesday August 7.
“Emergency services are currently on scene and the public are advised to avoid the area, where possible.”
Emergency services including police, ambulance and the fire service are on the scene.
Drivers have been urged to use an alternative route if possible and avoid the area.
Drivers should allow for extra traffic and delays on alternative routes, especially through Bilston and Straiton.
Traffic is also exceptionally busy on the A720 City Bypass.