The invention relates to a well drilling vibrator of a horizontal well. The well drilling vibrator comprises a shell, a piston head, a disc spring, an upper shoulder, a fixed sleeve, a sliding shaft, a valve shell, a spring, a central pipe, a sliding valve body, a nozzle, a water hole, a second flow passage, a valve plug, a first flow passage and a fixed valve seat, wherein the shell is in threaded connection with the fixed sleeve; the sliding shaft is arranged in a central hole formed by the shell and the fixed sleeve; the fixed sleeve is matched with the sliding shaft through a spline; the fixed valve seat is fixedly arranged in the valve shell through thread; the valve plug is matched with the fixed valve seat; when water pressure at the upper end of the valve plug reaches a limited value, the valve plug is separated from the fixed valve seat, the spring is compressed downwards by the central pipe, and drilling fluid flows by the first flow passage and then flows by the second flow passage; when pump capacity changes, the water pressure at the upper end of the valve plug also changes, the compression of the spring changes, the distance from the valve plug to the fixed valve seat changes, the flow area of the first flow passage also changes, and pulsating pressure is caused to change the compression amplitude of the disc spring by the upper shoulder of the sliding shaft, so that vibration is caused.