The invention discloses a horizontal well steam injection method and a horizontal well steam injection system. The method comprises: analyzing the flowing law of fluid for horizontal well steam injection, track of well bore of a horizontal well, steam injection amount of segmental horizontal well steam injection, and area of a steam injection hole; determining the positions of the steam injection valves in a horizontal well steam injection pipe column, steam injection amount and steam injection aperture according to the result of the analysis and a well temperature profile test curve; arranging the steam injection valves in the horizontal well steam injection pipe column according to the determined positions, steam injection amount and steam injection aperture, and performing the horizontal well steam injection by using the horizontal well steam injection pipe column. In the invention, the difficult steam injection execution problem caused by overlong horizontal well section, nonuniform horizontal well section steam injection and horizontal well screen pipe completion in the prior art is solved, uniform horizontal well steam suction can be realized and thus, the production of a thickened oil horizontal well is improved.