The invention relates to the technical field of engineering, and in particular relates to a soil nail grouting head device and a construction method thereof. The soil nail grouting head device comprises a soil nail steel bar, and a steel sleeve which sleeves the outer side of the soil nail steel bar, wherein the soil nail steel bar comprises a soil nail hole rod body; a centering bracket is fixedly clamped at the outside of the soil nail hole rod body; a polyethylene pipe is arranged at the outer side of the soil nail hole rod body; an L-shaped slurry stabilizing pipe is connected to the outside of the steel sleeve; a grouting hose is connected with the polyethylene pipe. The construction method of the soil nail grouting head device comprises the steps of preparing for construction, mounting the steel sleeve, drilling a hole and cleaning the hole, mounting the soil nail steel bar, mounting the slurry stabilizing pipe, grouting at low pressure, and supplementing the slurry in a manner of grouting under gravitational force. According to the soil nail grouting head device and the construction method thereof, the advantages of grouting through a low-pressure grouting method and supplementing the slurry in a manner of grouting under gravitational force are combined, so that the circumstances that slurry cannot be uniformly grouted in the soil nail hole during being grouted and the slurry overflows when the soil nail hole is nearly fully filled with the slurry can be avoided; meanwhile, the supplemented slurry level can be conveniently observed, and whether the soil nail hole is full of the slurry can be determined; the soil nail grouting head device has the advantages of being simple to operate, and low in grouting cost; and the grouting quality is ensured.