We propose a simple, efficient, and accurate analytical model for calculating the energy efficiency of a granular double-layer X-ray scintillator with a diffuse reflection layer, based on the first-order approximation of the radiative transfer equation by considering boundary conditions and exponential characteristics. Using the analytical model, we successfully analyze the characteristics of the double-layer X-ray scintillator, such as diffuse reflectance, transmittance, collection efficiency, and energy efficiency. We also suggest a design strategy for the high-energy-efficiency X-ray double-layer scintillator considering high diffuse reflectance and satisfaction of the target spatial resolution. Using the X-ray absorption ratio and the collection efficiency of the double-layer scintillator, the energy efficiency of the double-layer X-ray scintillator is calculated to achieve the best performance in terms of image brightness. Through calculation, we obtain the design of a double-layer X-ray scintillator with an energy efficiency of 8.7% with a computation time of less than a second.