The invention discloses a hydraulic top driving device of a drilling machine. The hydraulic top driving device of the drilling machine mainly comprises a tackle, a power head, a floating device, a lifting hydraulic cylinder and a braking device. A lifting plate of the tackle is connected together with a shell of the power head through a reamed hole bolt; a gooseneck pipe of the power head can penetrate out from a hollow shaft sleeve of the tackle; the lifting hydraulic cylinder is installed between the tackle and the power head; the power head is connected with the floating device through a drilling rod buckle, and the braking device is installed between the power head and the floating device. The hydraulic top driving device of the drilling machine has the advantages that a spindle is directly driven by a hollow motor, a reduction gearbox is omitted, the structure is simple, the fault rate is lowered, and the efficiency of the drilling machine is improved; the floating device is simple in structure, abrasion of the drilling rod buckle is lowered when the buckle is assembled and disassembled, and therefore a drilling tool is protected; the braking device is efficient, reliable and capable of achieving directional drilling.