A mobile device for measuring radon in groundwater, the mobile device comprising: a main body provided to be movable by a user and constituting an overall body; a groundwater sample chamber provided inside and outside the main body and storing a water sample collected from the groundwater; an air pump provided inside the main body, and configured to inject air into the water sample stored in the groundwater sample chamber at a constant pressure, and generating air bubbles; a buffer chamber provided inside the main body and configured to temporarily store the air bubbled by the air pump; a radon measurement part provided inside the main body, and configured to suction the air temporarily stored in the buffer chamber, detect in real time alpha particles present in the buffer chamber, output a predetermined alpha particle detection signal, count for a measurement time preset based on the output alpha particle detection signal, and calculate a radon concentration measurement value; a display provided to be exposed to an outer surface of the main body and configured to display the radon concentration measurement value calculated from the radon measurement part on a display screen; and a control part provided inside the main body, and configured to control an operation of the air pump, the buffer chamber, and the radon measurement part so that the air bubbled by the air pump is moved to the radon measurement part via the buffer chamber, and configured to control an operation of the display so as to quantify the radon concentration measurement value calculated from the radon measurement part and display the radon concentration measurement value on the display screen, characterized in that the buffer chamber is provided with a plurality of partitions installed therein for increasing an inner surface area in order to maintain moisture in the air bubbled by the air pump at a constant humidity. a main body provided to be movable by a user and constituting an overall body; a groundwater sample chamber provided inside and outside the main body and storing a water sample collected from the groundwater; an air pump provided inside the main body, and configured to inject air into the water sample stored in the groundwater sample chamber at a constant pressure, and generating air bubbles; a buffer chamber provided inside the main body and configured to temporarily store the air bubbled by the air pump; a radon measurement part provided inside the main body, and configured to suction the air temporarily stored in the buffer chamber, detect in real time alpha particles present in the buffer chamber, output a predetermined alpha particle detection signal, count for a measurement time preset based on the output alpha particle detection signal, and calculate a radon concentration measurement value; a display provided to be exposed to an outer surface of the main body and configured to display the radon concentration measurement value calculated from the radon measurement part on a display screen; and a control part provided inside the main body, and configured to control an operation of the air pump, the buffer chamber, and the radon measurement part so that the air bubbled by the air pump is moved to the radon measurement part via the buffer chamber, and configured to control an operation of the display so as to quantify the radon concentration measurement value calculated from the radon measurement part and display the radon concentration measurement value on the display screen, characterized in that the buffer chamber is provided with a plurality of partitions installed therein for increasing an inner surface area in order to maintain moisture in the air bubbled by the air pump at a constant humidity.