The invention discloses a prediction method for the gas emission quantity of an inclined coal seam horizontal layer mining working surface. The prediction method comprises the following steps that S001, the gas prediction basic data of the working surface are investigated and calculated; S002, the gas emission quantity q<1> of a mining layer is calculated; S003, the gas emission quantity q<2> of an adjacent coal seam, adjacent to a mining coal seam, of a stoping working surface is calculated; S004, the pressure relief gas emission quantity q<3> of the lower portion of the stoping working surface is calculated; and S005, the gas emission quantity q of the mining working surface is calculated, and q=q<1>+q<2>+q<3>. The invention further discloses a prediction system for the gas emission quantity of the inclined coal seam horizontal layer mining working surface. According to the provided prediction method and system for the gas emission quantity, the gas emission quantity prediction model property of the horizontal layer mining working surface of the steeply inclined and extremely thick coal seam is established through the prediction method and system, and the defects of existing techniques are compensated.