Abstract: In the InSAR system based on formation-flying satellites，because the relative position of each satellite participated in formation moves continuously and need a credible way to measure accurately, which is Interaerocraft baseline measurement and baseline estimation. In spite of anyway, the error must be exist. On the processing of the InSAR, baselines are important parameters, and vitally influence the precision of the height of ground point. Baseline is important in InSAR processing, and which is the important factor and decide the chose of interferometric image twins, and be helpful to flat domino effect. Baseline precision decides the last result for InSAR processing. No hi-precision baseline, and no hi-precision DEM. Baseline detection is one of important ways to get high-precision baseline. As yet, the work has not been done in our country, so there are not experiences to use for reference and there is little record in the overseas. This paper studies the baseline detection in the formation-flying satellites. Baseline detection deal with the baseline parameters which is hi-dynamic and distributed by using the hi-precision ground area data. By analyzing and comparing many baseline accurately estimation model, I deduce an advanced-model of baseline estimation based on baseline linear fitting. The tests of baseline detection have been done by using the ERS real data. The result shows the baseline precision has been advances greatly, so the way has practicality very much.