We calculate the resistivity of the insulating layer in a tunnelling-magnetoresistive (TMR) magnetic head byusing the Landauer-Büttiker formula with a fast Green function method, where a recursive process with a faster simulation speed and higher accuracy is carried out to substitute the inversion of Greens matrix. A tight-binding model with an energy barrier △ E is utilized to simulate the magnetoresistive tunnelling junction in the TMR head. The resistivity of the insulating layer is 2.6 × 105μΩcm with two oxygen-ion layers and △E = 2.5 eV, which agrees with the experimental data.