Atom-interferometric gravitational-wave (GW) space observatory (AIGSO) is a mission concept mainly aimed at the middle-frequency (0.1-10 Hz) GW detection. AIGSO proposes to have three spacecrafts in linear formation with extension of 10[Formula: see text]km. The three spacecrafts need to maintain [Formula: see text] constant arm-length formation. In this study, we address the issue of orbit design and thruster requirement. The acceleration to maintain the formation can be designed to be less than 30[Formula: see text]pm/s²and the thruster requirement is in the 30[Formula: see text]nN range. Application to other arm-length-maintaining missions is also discussed.