The utility model discloses an underground generator for a drilling instrument and belongs to the field of drilling devices. The generator comprises a generator body (1), a magnetic coupler (2), a mud turbine (3) and a connecting rotary shaft (4), wherein the magnetic coupler (2) is arranged in the mud turbine (3), and two ends of the connecting rotary shaft (4) are fixedly connected to the generator body (1) and the magnetic coupler (2) respectively. The generator is simple in structure, low in cost, long in service life, safe and reliable.