Abstract: Constant measurement baselines are very important to satellite formations. So this work stresses on conditions for the constant measurement baselines. Based on the characteristics of relative motion of satellite formations and the definition of baselines in satellite formations, this article firstly studies the relationship between baselines and formation configuration parameters. A criterion called Stabilization Level is then proposed, which can preferably indicate the stabilization of varying baselines. Under the same stabilization level and orbit period, stabilization periods of time of the maximum horizontal and vertical baselines in three particular formations are analyzed. The three formations are at a certain extent similar to foreign formations： Interferometric Cartwheel, Interferometric Pendulum and CarPe. Results show that the three formations cannot meet the requirement of constant horizontal and vertical baselines synchronously. Therefore, the article researches on the conditions for the constant measurement baselines and draws a conclusion： the sufficient and necessary conditions are three constraint formulas for constant horizontal baseline and two for constant vertical baseline. Another important conclusion obtained from the above one is that the formation by two satellites cannot satisfy the above requirement. At end of the article, a formation by three satellites is designed according to the above research, which can attain the constant horizontal and vertical baselines simultaneously and validate the above research