It is shown theoretically and by particle- in-cell simulations that the plasma density modulation produced by laser in underdense pre-plasmas can serve as a Bragg reflector, which results in phase reflection occurring at low-density plasma area. The phase reflection can influence the laser propagating in plasmas, such as the vacancy of laser field induced by two laser pulses counter-propagating in plasmas. It is also shown that the period of phase reflection and the reflectivity are related with the plasma density, thickness, intensity and the duration of laser.