The invention provides a sampling slicer for HDPE membrane weld inspection and a slicing method thereof. The sampling slicer for HDPE membrane weld inspection comprises a bracket, a manual pressure rod and a blade unit. The blade unit is arranged in the middle of the bracket. The blade unit comprises an upper blade unit, an upper knife rest, a lower blade unit and a lower knife rest; the upper blade unit is mounted on the upper knife rest, the upper knife rest is connected to the upper portion of the bracket through a spring, the lower blade unit is mounted on the lower knife rest, the lower knife rest is mounted on the lower portion of the bracket, and the upper blade unit and the lower blade unit are opposite to each other and are arranged in a spaced manner; one end of the manual pressure rod is hinged to the upper portion of the bracket and the manual pressure rod is located right above the upper blade unit, a circular portion which enables the upper knife rest to press down is arranged on one end of the manual pressure rod, the radius of the circular portion is greater than the gap between the upper blade unit and the lower blade unit, and the manual pressure rod is pressed down to engage the upper blade unit and the lower blade unit so as to achieve a purpose of cutting an HDPE membrane. The sampling slicer provided by the invention is simple in structure and simple to operate, the sampling speed is increased, and the manpower and material resources are greatly saved.