This paper presents a method for simulating daylight illuminance of indoor scenes with astronomical calculation and computer graphics rendering. The rendered phenomena look like real sunlight streaming into an interior through an outside window. At first, users interactively input date, location of the observed room, and turbidity of atmosphere. With these parameters, we can then calculate the solar declination angle, azimuth, and luminous intensity in terms of formula derived from astronomy and geography. Given the luminous intensity in a room at a specific time, we can also determine the room's energy and the spots receiving sun's direct irradiation. We take sunlight from window as parallel lines and find out meshes that have intersections with those lines, and then distribute the sun's energy to those meshes. Thus we can simulate indoor scenes with radiosity algorithm. Finally results are shown on daylight simulation.