The advanced data storage technology is important to information era. Among all sorts of solutions of high-density storage, near field optical disc technology (NFOD) is high speed and mass storage technology with excellent aptitude and future, and it is the focus of data storage research field in the pioneer technology. Today the research institutes all over the world are speeding their research on NFOD. By using the techniques of solid immersion lens (SIL) and super-low-flying system, it can achieve not only super-high recording density that can be much more higher than traditional optical disks but also the hard disks. In order to improving near-field coupling efficiency of SIL-TO-DISK, the SIL must keep sub micron flying height from the disk, so it is necessary to discuss the research process of real time method to measure SIL-TO-DISK for super-low-flying system. This paper analyses technique foundation and characteristc and its key problem for the flying height measurement, the paper studies several practicing plan of real time measure the clearance even when the SIL-DISK spacing is down to nanometer level for example, relative light intensity method, the capacitance displacement sensors, effective refractive index method for frustrated total reflection, and compare the characteristic and precision of those approach.