The invention relates to a pore water pressure testing method for a low-water-content compact rock soil medium. The method comprises the steps of measuring a relative humidity of a low-water-content rock soil sample and determining a first balance formula according to a chemical potential balance principle; determining a second expression formula according to a first expression formula and relationship between a current-state actual pore water pressure and measured pore water pressure; removing pore water in the rock soil sample out to form a balance solution, measuring a relative humidity of the balance solution and determining a second balance formula according to the chemical potential balance principle; combining the second expression formula with a third expression formula to calculate out the measured pore water pressure. The pore water pressure testing method has simple and effective operation method and can accurately obtain pore water pressure of the low-water-content rock soil medium in a shorter time.