It is shown theoretically and by particle-in-cell simulations that the interferi ng fields of the incident and reflected laser pulses can produce deep plasma den sity modulations in underdense pre-plasmas.This occurs within a few tens of lase r cycles if the incident laser pulse is at moderate intensities such as 10 15W／cm2Such density modulations can serve as a Bragg reflector ,which results in phase reflection from underdense plasmas.Because the deep dens ity modulations are just a period of half a wavelength in the plasma，the reflec ted wave reaches the maximum by resonance with a reflectivity up to near 100％.T he phase reflection can also be observed from the plasma with a linear density p rofile，which can significantly reduce the laser absorption by fusion targets.