The invention relates to a flat ending method of monocrystal silicon and a preparation method of monocrystal silicon. The flat ending method comprises the steps of closing an automatic temperature control system, and stopping the rise of a crucible; manually heating up 10-20 DEG C, reducing crystal lift speed to 0.1-0.4mm/min, maintaining crystals to continuously grow for 20-30min, and enabling the ends, contacted with a solid-liquid interface, of the crystals to grow on a plane instead of on a concave surface; closing an automatic crystal lift speed control system, manually reducing the lift speed to less than 0.1mm/min, maintaining the crystals to continuously grow for 20-30min, and enabling the ends, contacted with the solid-liquid interface, of the crystals to grow on a convex surface instead of on the plane; dropping the crucible by 20-50mm at a time, enabling the crystals to get away from molten silicon liquid level in the crucible, and reducing the rotating speeds of the crystals and the crucible; cooling the crystals, and continuously lifting the crystals to obtain a product. After the flat ending method of the monocrystal silicon is adopted, the consumption of molten silicon is reduced, the preparation cycle of the monocrystal silicon is shortened, the length and weight of a qualified monocrystal silicon product are improved, and the pass rate of finished products is increased.