If the situation of quantum gravity nowadays is nearly the same as that of quantum mechanics in its early time of Bohr and Sommerfeld, then a first-step study of the quantum gravity under Sommerfeld's quantum condition of action might be helpful. We present the spectra of quantum Schwarzschild black hole in non-relativistic quantum mechanics. It is found that the quantum of area is 8π3l2p, the quantum of entropy is 2π3kB, and the Hawking evaporation will cease when the black　hole reaches the ground state　m=123mp. A very natural conjecture is that the constituents of dark matter may come from the remnant (relics) of primordeal black hole with life time smaller than that of the universe.