Abstract: Some problems exist in the defect detection and judgment of the inner and outer pipes of insulated tubing during the heavy oil thermal recovery. This paper proposes a magnetic flux leakage (MFL) detection method of insulated tubing under DC exciting mode. Based on the principle of DC exciting magnetic flux leakage detection and Maxwell's equations, the detection structure of insulated tubing is simulated using the finite element simulation software. The experimental system detecting the defects of inner and outer pipes is established. The values and distribution characteristics of B_x and B_y components of the defects of inner and outer pipes are analyzed. The research results show that the MFL detection method under DC exciting mode can realize the detection of inner and outer pipes defects and distinguish the defects signals beneath 30% depth.