Abstract: Based on the atmospheric scattering principle,the paper summarizes the color and brightness characteristics of the Sun and sky in clear days at different time,and accordingly a real-time simulation to the characteristics is made in a day.We simulate the atmospheric Rayleigh scattering by sampling the gradients map and propose a simplified index formula to simulate the atmospheric Mie scattering.Furthermore,the high dynamic illumination of the Sun,sky and ground with auto-key high dynamic range lighting is taken into account in the simulation.The analysis to the experimental results shows that our system can achieve both real-time performance and high fidelity effects.