In order to implement multi-gray-level holographic storage using a binary spatial light modulator,a data page to store is divided into a series of equivalent binary data pages. The recording time is also split into a series of time slices corresponding to the binary data pages. During each time slice,a binary data page is uploaded by the binary spatial light modulator into the object beam which interferes with the reference beam. A series of holograms are recorded in the material. It is proved theoretically that the gray-level distributions of the reconstructed data page are equal to those of the original data page. An experiment of four-gray-level storage is carried out to verify its feasibility. The multi-gray-level storage method can enlarge the capacity of the stored data page. The dynamic range of the material is fully made use of. And the storage density and readout data rate are both improved.