Abstract: The Direct Current Injection (DCI) is an effective method to replace the free field illumination in the air craft level high intensity radiated field test. According the equivalence of the current response between the two methods in the cable bundle port , this paper establishes the mathematics relationship between radiation field and direct injected current through the transfer functions. On this basis, mathematics simulation method is applied to simulate and calculate current transfer function of every cable bundle port in an airframe model, and further, the direct inject current value is established. The method presented in this investigation has important direction in the whole aircraft level HIRF test.