The invention relates to a device and a method for preparing soil mass for simulating a slope seabed. The device is characterized by comprising a transparent soil box, wherein left and right two sidewalls of the soil box are vertically provided with support structures respectively; each of the support structures is provided with an adjustable crossbeam on a horizontal level; a cant beam is arranged on the two crossbeams; sliding rails are arranged along the cant beam, and is provided with a slide block which is connected with a scraping plate; the front and rear two sidewalls of the soil box are provided with a horizontal bracket respectively; one end of the each horizontal bracket is provided with a fan-shaped support plate which is provided with an arc chute, while the other end is provided with a support; the two supports are hinged with the sliding rails respectively, and the other ends of the two sliding rails are arranged into the arc chutes on the corresponding side, and locked by a locking device; a sand sprinkling vehicle is erected on the two sliding rails; a motor is arranged outside the soil box; an output end of the soil box is provided with a hoisting wheel, and the hoisting wheel is fixedly connected with a stay rope; and the other end of the stay rope goes around two fixed pulleys which are arranged on one side of support structure sequentially, and the stay rope hangs one of the slide block and the sand sprinkling vehicle.