The invention belongs to a diamond bit special for an underground horizontal drilling device of an oil reservoir of an oil field, and solves the problems of low efficiency, serious wearing and the like of the conventional special hard alloy bit for drilling permeable sandstone and a metal casing. The diamond bit consists of a diamond blade 1, a bit body 2 and a blade base 3, wherein the side face of the bit body 2 is provided with symmetric chip removal grooves 4 which are linear or helical grooves; and the top end face of the bit body 2 is symmetrically provided with liquid outlet holes 6 which are cylindrical liquid outlet holes. The diamond bit is mainly used in underground horizontal drilling of the oil reservoir and has the advantages of simple and novel structure, good chip removal, high cutting efficiency, long service life and the like.