This paper focus on the problem of asymmetrical evaluation level caused by the difficult of discrimination of internal and external defects of MFL inspection signals of oil well pipes to analyze the difference of change trend between the peak-to-peak value of normal detection signals of internal and external defects and the liftoff distance z. The liftoff characteristic is verified through experiments that the liftoff effect of internal defects has a gently change rate but the external has a sharp change rate. A double liftoff inspection method was further proposed, that was, there were double layers magneto sensors arraying in line in the normal direction to accomplish grads measure of two special spaced points in the liftoff direction. Then made cross correlation contrast of detection signals. Result in a new evaluation using the damping decrement of the peak-to-peak of inspection signals in the direction of liftoff. Finally, the unitary damping decrement as an evaluation value used for direct judgment was presented and confirmed by experiments, indicating that the proposed approach was of feasibility.