A backlight module, a spatial light modulator, a holographic display device and a holographic display method therefor are disclosed. The backlight module includes: a light source; an optical conversion element arranged to face the light source and configured to convert a light beam emitted by the light source into coherent collimated light; and a light switch layer disposed at a side of the optical conversion element away from the light source. The optical conversion element includes a plurality of sub-optical conversion regions. The light switch layer includes a plurality of sub-light switches, the plurality of sub-light switches and the plurality of sub-optical conversion regions being disposed in one-to-one correspondence.