The optimal measuring zone under the optimal arrangement of four laser tracking interferometers is discussed, and the distribution of PDOP of the measuring points in 3D space is also given. The relationship between system arrangement and the self-calibration is also studied by using of computer simulation. The results show that the nominal setup of all the optimal measuring arrangements plays the least role in the process of the system self-calibration and has the minimal value of PDOP of the measuring points. So, it is finally reached out that the nominal setup is just the optimal arrangement of the whole system.