The present invention relates to a diamond Si polycrystalline lockable tetrahedral knife and a manufacturing method thereof. Si is used as binding agent to make polycrystalline material which is combined with diamond micro powder into the lockable tetrahedral knife. The method comprises the steps as follows: the diamond micro powder is sintered and combined with Si compactly by taking Si as the binding agent under high temperature and high pressure (the temperature is 1573 DEG C, and the pressure is 5.8GPa) to generate another new material diamond SiPCD (polycrystalline Si); the diamond SiPCDis cut into a tetrahedron by a laser cutting machine; and the cut tetrahedron is carried out accurate grinding and made into the tetrahedral knife. The diamond Si polycrystalline lockable tetrahedralknife is made from the diamond SiPCD; the size of the tetrahedral knife is 9.8 * 9.8 * 1.6, 2.0 and 3.2; the center is provided with a centre hole with the diameter of 3-6; and the tetrahedral knife is the shape of the tetrahedron with arc or without the arc. The present invention has the beneficial effects that the present invention replaces the hard alloy cutting tool used in the traditional milling cutter industry, and at the same time, the present invention also prolongs the service life of the milling cutter greatly.