Abstract: Ambient dose equivalent is the calibration quantity and type inspection quantity of environmental and workplace radiation monitoring instrument. The conversion coefficient from air kerma to ambient dose equivalent recommended in ISO 4037-2 is suitable for “matched reference fields”. Due to the different X-ray tube type and overall structure of X and γ-ray irradiation devices in different standard laboratories, the photon spectrum distribution is different from the nominal reference field, and then the conversion coefficient is deviated. In this paper, MCNP5 program is used to establish the model of irradiation device, and the corresponding conversion coefficients of 15 keV~1.5 MeV single energy photon and N-40~N-200 narrow spectrum series are calculated. The ambient dose equivalent rate at reference point of radiation field is determined by self-developed H*(10) secondary standard ionization chamber and compared with the conversion coefficient method. The results show that the relative deviation between the calculated and recommended value of conversion coefficient in ISO 4037-3 is within ±4%. The relative deviation between the secondary standard chamber method and conversion coefficient method is within 3%, which prove that the Monte Carlo method for calculating H*(10)/Kα conversion coefficient is feasible. For “unmatched reference radiation field”, this method can be used to determine ambient dose equivalent.