The invention provides an SAGD (steam assisted gravity drainage) horizontal well system and a starting method thereof. The SAGD horizontal well system comprises two horizontal wells which are an upper horizontal wall and a lower horizontal wall, wherein first steam injection pipes and second steam injection pipes are arranged in both horizontal wells. The SAGD horizontal well system further comprises at least one water injection unit, wherein the water injection unit is communicated with at least one horizontal well through the first steam injection pipes or the second steam injection pipes. The at least one water injection unit is arranged in the SAGD horizontal well system, the reservoir stratums in the SAGD horizontal wells are changed by means of pressure of water, the porosity and the permeability of the oil layer of the near wellbore area are improved, communication of the SAGD horizontal wells can be realized in shorter time, the starting time of the SAGD horizontal well system at the earlier stage is greatly shortened, besides, the communication between the steam injection well and a production well is improved, and accordingly, the oil production of the SAGD horizontal wells in the later period is increased.