From the perspective of micro-nano optical fiber coupling， an optical fiber sensor is designed by using single mode optical fiber 2×2 coupling. By changing the tapering parameters of optical fiber coupler， the over-coupling state is reached. At this time， the over-coupling structure of optical fiber itself is affected by the environment， so that the wavelength of light has a higher response to the change of external temperature and strain， realizing the measurement of temperature and strain. In the experiment， optical fiber over-couplers with overcoupling length of 22 000 μm and 22 500 μm are tapered， and their transmission spectra were compared. The temperature and strain sensing tests results show that the lowest wavelength response to temperature is about 104 pm/°C， and the lowest wavelength response to strain is about $-$21 pm/με.