The invention discloses a planar grouting pad water gushing port flow control device and a construction process thereof. The flow control device comprises a grouting pad arranged at a karst outlet of a karst pipeline. A water diversion pipe is arranged in the grouting pad. One end of the water diversion pipe communicates with a water gushing channel. The other end of the water diversion pipe extends out of the grouting pad and is provided with a valve. According to the construction process, the water gushing flow can be controlled by arranging the grouting pad and the valve at a water gushing port; along with decreasing of the water outlet amount of the water gushing port, the water flow velocity in the karst pipeline is reduced gradually on the condition that the total water passing volume in the flowing water karst pipeline is unchanged, and the retention rate of blockage grout in the pipeline is increased so as to realize successful blockage of the karst pipeline.