The invention belongs to the field of building materials and particularly relates to a non-cement green mortar by using the steel slag as the cementing material and the fine aggregate and a preparation method thereof. The non-cement green mortar comprises the following components in weight part: 30-50 parts of steel slag powders, 50-70 parts of mineral powders, 200-350 parts of steel slag sands, 4-7 parts of activator, 1-4 parts of a water reducing agent, 3-6 parts of a dispersing agent, 0-0.4 parts of a retarder, 0.0002-0.001 part of a defoamer, and 30-40 parts of water. The non-cement green mortar is good in working performance, automatic in leveling effect, adjustable in setting time with the range of 0.5-8 hours, high in early-late strength, and excellent in impermeability and resistance to sulfate attack. The 7d breaking strength of the non-cement green mortar is larger than 8 MPa, and the compression strength thereof is larger than 40 MPa. The 28d breaking strength of the non-cement green mortar is larger than 9 MPa, and the compression strength thereof is larger than 50 MPa. Therefore, the non-cement green mortar can be widely applied to the engineering practice.