Abstract: To decrease drilling fluid contamination caused by asphaltic heavy oil influx,a hardening agent LYJ-5 was developed based on the chemical theory that asphalt get harder after oxidation.Solidifying test was conducted in drilling fluid environment,the effect of temperature and pressure on solidifying performance was evaluated,then the selected hardening agent was applied in oilfield.LYJ-5 could increase the softening point of asphalt from 52 ℃ to 127 ℃ and the softening point temperature was raised by 52 ℃ in drill fluid and 75 ℃ without drilling fluid respectively.Its application in Well F17 showed the contaminated drilling fluid volume per hour decreased by 19.15% after adding LYJ-5,solidifying and accumulation of asphalt was observed at outlet,making wire line logging and casing running smooth.It was concluded from comprehensive study that LYJ-5 could increase the softening point of asphaltic heavy oil significantly,reduce the drilling fluid contamination caused by asphaltic heavy oil,and could guarantee safety drilling in asphaltic heavy oil bearing formation.