Based on the principles of earth rotation and revolution, the effects of the obliquity of the ecliptic plane, solar declination, longitude, and solar elevation angle on the formation of the sunlight line on a dark tube sunshine recorder are analyzed. The principles of sunlightline location displacement with seasonal variation are described, and the regularities of sunlightline formation at spring equinox, autumnal equinox, winter solstice and summer solstice are discussed. Under the condition of the fixed earth motion trajectory, the daytoday sunshine path line can be obtained. With the theoretical basis, the software for sunlight line formation are designed and introduced.