The invention discloses a device for measuring the air entry value of unsaturated soil. The device comprises a cover plate, pressure chamber sidewalls, a pottery clay plate, a substrate, a water inlet pipe, a water pressure-free water discharge measuring pipe and a rack, wherein the cover plate, the pressure chamber sidewalls and the substrate form a closed test space; an air inlet pipeline communicated with a pressure air source is arranged on the cover plate; the pottery clay plate is fixed on the substrate; a water inlet pipeline communicated with the water inlet pipe is arranged on the central part of the substrate; a water outlet pipeline communicated with the water pressure-free water discharge measuring pipe is arranged on one side of the substrate; the water inlet pipe and the water pressure-free water discharge measuring pipe are arranged on the rack on the two sides, respectively; isometric helical line type water discharge grooves are formed in the substrate. According to the device, bubbles at the bottom of the substrate are automatically discharged along with water flow through the water discharge grooves, and errors caused by the bubbles are not counted into a measured water discharge value, so that the accuracy of a test result is ensured.