The utility model discloses a high-pressure on-line grease injection device for a valve. The device comprises a grease injection machine, a first isolating valve, a first emptying valve, a first pressure gauge, a one-way valve, a second emptying valve, a second pressure gauge, a second isolating valve and an oil conveying pipe, and the grease injection machine, the first isolating valve, the firstemptying valve, the first pressure gauge, the one-way valve, the second emptying valve, the second pressure gauge and the second isolating valve are sequentially communicated through the oil conveying pipe. The device is simple in structure and convenient to use, and can be made of existing materials. When the grease injection valve is used, the second isolation valve is connected with the greasenipple of the valve to be injected with grease, so that the grease can be safely and effectively injected into the valve under the condition that normal production under pressure is not influenced, and particularly, the grease can be safely and effectively injected into the valve aiming at a valve leaked out of the grease nipple, so that the valve is maintained in time. Therefore, the problems that the grease nipple leaks outwards and the pressure-hidden valve cannot safely and effectively inject grease for maintenance are solved.