The present invention relates to a compound formed by entinostat as shown in formula (I) and an acidic counterion. Compared with the known solid form of entinostat, the compound involved has advantages in terms of solubility, stability, etc. The present invention also relates to a crystal form of the compound and a preparation method therefor, a pharmaceutical composition thereof and the use thereof in the preparation of a drug for preventing and/or treating a disease associated with differentiation or proliferation.