The invention discloses a preparation method of an N-doped polysilicon ingot. The preparation method comprises the following steps of: putting polysilicon and silicon nitride nanopowder with the grain size of 1 to 100nm in a quartz crucible and charging, vacuumizing a furnace room and heating to 1420 to 1550 DEG C, and preserving the temperature till mixture is fused completely; gradually reducing the cooling speed in 1 to 10 DEG C/min in a furnace till reaching 1350 to 1420 DEG C; then naturally cooling the temperature in the furnace room to the room temperature; taking the silicon ingot outand crushing into small pieces to obtain master alloy silicon blocks; putting mixture of the master alloy silicon blocks, the polysilicon material and electrical activity dopants in the quartz crucible, vacuumizing the furnace room and heating to 1420 to 1550 DEG C to obtain a fused silicon material mixture; and enabling the fused silicon material mixture to solidify to obtain the polysilicon ingot. According to the preparation method of the N-doped polysilicon, the invention can carry out N-doping by adopting conventional devices and has even N-doping, high N-doping concentration, simple entire process and low cost. In addition, the invention also discloses a preparation method of an N-doped monocrystalline silicon ingot.