The small satellite Flying Laptop is the first satellite which is developed by the Institute of Space Systems at the Universität Stuttgart within the "Stuttgart Small Satellite Program". In this paper the detailed system design of this satellite is described which enables its mission objectives: new technology demonstration and scientific earth observation. The Flying Laptop is the test bed for an on-board computer with a reconfigurable, redundant and self-controlling high computational ability which is based on a field programmable gate array. The attitude control system with pointing knowledge of better than 7.4 arcseconds makes it easy to achieve maximum ground sample distance of 25m for Multi-spectral Imaging Camera System and 100m for Thermal Imaging Camera System of payload, and the high speed communication channel in Ka-band up to 500 Mbit/s allows enough data transfer budget, which is exceptional for small satellites. All input/output signals of the on-board computer, in total more than 200 lines, include interfaces of RS422, LVDM, M-LVDS, TTL and I2C. The harness system is developed using mock-up model of the whole satellite in its original size. The introduced operational scenario and defined attitude control modes ensures the validity of the system design. Copyright IAF/IAA. All rights reserved.