The invention discloses a mechanical screening device. The mechanical screening device comprises a hemispherical seat, a balance ball is fixedly connected to the middle position of the inner surface of the hemispherical seat, a partition plate is movably connected to the position, close to the balance ball, of the inner surface of a screening barrel, a fluctuating mechanism is movably connected to the side, away from the balance ball, of the partition plate, and a spherical soft plate is movably connected to the side, away from the partition plate, of the fluctuating mechanism. An inclination angle mechanism is movably connected to the position, located on the partition plate, of the outer surface of the screening barrel, screening holes are formed in the position, located above the spherical soft plate, of the outer surface of the screening barrel, and a water injector is fixedly connected to the middle position of the top of a top cover. The invention relates to the mechanical technical field. Water flow is used for cleaning away dust on the surfaces of plastic particles, dust is prevented from influencing the weight of the particles, the particles are effectively layered through different water buoyancy degrees caused by different shapes of the particles, meanwhile, the device obliquely rotates, the particles are layered and discharged, and the particle screening quality is guaranteed.