The A15 between Quarrington and Silk Willoughby has been closed due to a fire involving 24,000 straw bales.
The blaze is near Greylees and Quarrington, and police are asking the public to avoid the area as well as advising nearby residents to keep doors and windows closed to avoid smoke entering their properties.
Five crews are in attendance after the straw stack fire was reported to Lincolnshire Fire and Rescue at 6.04pm.
Two crews from Sleaford and firefighters from Billingborough, Billinghay and Metheringham are at the scene along with a water carrier from Holbeach.
They are using a jet and three hose reels to tackle the flames.