The invention relates to an energy-saving horizontal mortar joint building method which is characterized in that a flaky heat insulating material with good thermal property is horizontally laid at the middle part of the mortar laying surface of a building block along the length direction of a wall, mortar is laid at two sides of the mortar laying surface of the building block, near the wall surface, and the mortar and the flaky heat insulating material form horizontal mortar joints together. By using the method of the invention for building horizontal mortar joints, the thermal property of the heat insulating wall body using building blocks as main materials is obviously improved, the mortar consumption and the waste are reduced, the weight of the wall body is relieved, the permeability resistance of the wall body is improved, and the construction is convenient and rapid.