An ultrahigh resolution refractive index sensor based on etched DFB fiber laser is introduced.The surrounding refractive index changes the phase shift of DFB fiber laser.The wavelength variation of DFB laser caused by phase shift is analyzed in detail.The wavelength difference of fundamental mode and higher order mode is found to offer self temperature compensation.The resolution of the sensor is 4.7×10-7 with a laser line-width of 100 kHz.