Officers had shut the eastbound carriageway of the A27. The second lane was open but police then had to shut it also after the vehicle caught fire a second time.
PC Tom Van Der Wee of Sussex Roads Policing Unit said: "Currently have the eastbound carriageway of the A27 at Tangmere closed just east of the Tangmere roundabout.
"Dealing with an agricultural vehicle which was on fire. Lane two was open however is closed temporarily due to it catching fire again."
He also confirmed that fire crews from Chichester are at the scene.
Sussex Roads Police said in response to PC Van Der Wee: "One lane is now open but traffic is very heavy on the #A27 eastbound between #Chichester and #Tangmere as a result. Expect to have the road open fully within the hour. #CV282"