The invention relates to a crop straw automatic smashing discharging soil fertilizing device and a straw soil fertilizing method. The device comprises a supporting plate, a cutting device and a discharging device. A square groove is formed in the right end of the supporting plate. The cutting device is installed in the middle of the supporting plate, and the discharging device is connected with the right end of the supporting plate; the discharging device comprises a stirring cylinder, a stirring connecting column, a stirring motor, a stirring rotating rod, a stirring chassis, a stirring sideframe, a discharging frame, a discharging mechanism, a discharging rotating plate and a rotating plate push rod; the discharging mechanism comprises a discharging motor, a discharging rotating shaft,a rotating shaft connecting plate, a discharging roller, a discharging guide plate, a guide plate spring, a positioning support, a positioning stretchable column and a positioning clamping plate. Theproblems that during returning and discharging of existing straw, straw needs to be cut and conveyed artificially, the straw can fall down during cutting and conveying, the straw and a nutrient solution are stirred not uniformly, the stirred straw needs to be thrown and sprayed artificially, and straw throwing and spraying are not uniform can be solved.