The invention belongs to the field of engineering machinery, and particularly relates to a quantitative spreading device for a straw-decomposing inoculant mixture. The mixture is quantitatively sent to a spreading plate and is spread to straw in the field through centripetal force produced during rotation of the spreading plate; the spreading plate is arranged below the device, a central spindle is arranged in a material box, and an auger is mounted outside the central spindle; the central spindle is used for controlling the spreading plate to spread the mixture; the auger is used for vertically lifting, conveying and mixing the mixture; a feeding port is formed in the lower end of an auger casing, a discharging port is formed in the upper end of the auger casing, and a mixture extraction port is formed in the middle; and the mixture enters the auger through the feeding port in the lower end of the auger casing, the auger drives most mixture to return to the material box from the discharging port of the auger casing, and the mixture circularly flows to achieve the stirring purpose. The quantitative mixture flowing out of the extraction port in the middle reaches the spreading plate along a guide pipe, so that purposes of mixture lifting, stirring, mixing and quantitative conveying are achieved. The quantitative spreading device for the straw-decomposing inoculant mixture has the characteristics of simple structure, uniformity in spreading, high production efficiency and the like.