Abstract: Since the butane deasphalting equipment was put into production in refinery of Henan Oil field. Engler viscosity of semi -bitumen is more than 200E (Kinematic viscosity reached 1500mm2/s) under 100℃ Owing to its viscosity is too high, if these semi - bitumens are used as fuels of a boiler, then its results are poor, but interfereing with normal production. Through large number tests and researches, we discover that the viscosity reduction rate under conditions of pressure 0.5MPa, temperature 400 -420 ℃ and time 20 - 40 min can reach 86 % - 90% in cracking reaction, results are tangible. Use of this way, we have completely solved the problem of semi - bitumen way out, and can economize blended thinning oil about 10 000t/a, achieved remarkable social and economic benefits.