Direct X-ray detectors based on semiconductors have drawn great attention from researchers in the pursuing of higher imaging quality. However, many previous works focused on the optimization of detection performances but seldomly watch them in an overall view and analyze how they will influence the detective quantum efficiency (DQE) value. Here, we propose a numerical model which shows the quantitative relationship between DQE and the properties of X-ray detectors and electric circuits. Our results point out that pursuing high sensitivity only is meaningless. To reduce the medical X-ray dose by 80%, the requirement for X-ray sensitivity is only at a magnitude of 10³ μCGy⁻¹⋅cm⁻². To achieve the DQE = 0.7 at X-ray sensitivity air from 1248 to 8171 μCGy⁻¹_air⋅cm⁻², the requirements on dark current density ranges from 10 to 100 nA⋅cm⁻² and the fluctuation of current density should fall in 0.21 to 1.37 nA⋅cm⁻².