A PN junction, a method for preparing a PN junction, and a modulator are disclosed. The PN junction includes a slab waveguide layer (401) and a plurality of waveguides (402) arranged on the slab waveguide layer (401) at intervals along a specific direction. The slab waveguide layer (401) includes P-type doped regions (401-1) and N-type doped regions (401-2) that are alternately arranged in the specific direction, and a carrier-depletion region (403) in a shape of S connected head to tail along the specific direction is formed at a boundary between the two types of regions. Each of the plurality of waveguides (402) arranged at intervals includes a P-type doped region (402-1) and an N-type doped region (402-2), and one carrier-depletion region (403) is formed at a boundary between the two regions. Projections of the P-type doped regions (402-1), the N-type doped regions (402-2), and the carrier-depletion regions (403) of the plurality of waveguides (402) arranged at intervals on a surface of a slab waveguide region basically coincide with projections of corresponding regions of the slab waveguide layer (401) on the surface of the slab waveguide region. The PN junction has high modulation efficiency and a low optical loss. The PN junction is implemented through ion-implantation doping at an inclined angle, so that preparation costs of the PN junction can be reduced.