The invention discloses a device for measuring water content of green sand and a water content acquiring method, relates to the field of casting detection and solves the problem of low water content measuring accuracy due to mutual influence between a resistor and a capacitor when a capacitor method or a resistor method is used to measure water content of the green sand. The device is composed of a signal source generating module, a 0-degree square wave shaping module, a water content probe sensor, a 90-degree square wave shaping module, a first phase sensitive detection module, a selective frequency amplifying module, a second phase sensitive detection module, a second low-pass filtering module, a first low-pass filtering module, an A/D conversion module, a STM32 system and a liquid crystal display module. The method includes: acquiring dielectric loss angle of the green sand by measuring quotient of two signals with phase difference of 90 degrees after low-pass filtering; acquiring water content according to an existing relation between the dielectric loss angle of the green sand and the water content. By the method, separate calculation of resistance value or capacitance value is avoided, and measuring accuracy of green sand water content is improved greatly.