The invention discloses a printer as well as a paper-accommodating assembly and a paper-used up detection mechanism thereof, wherein the paper-accommodating assembly comprises a turning plate, an elastic element, and a sensor, wherein a first arc groove is formed near a rotating center on the upper surface of the turning plate, and a second arc groove is formed far way from the rotating center onthe upper surface of the turning plate; the elastic element supplies rotating torque to the turning plate so that the turning plate is turned from an initial position to an upturning position during the printing and consuming of a paper roll; the sensor is used for detecting papers; and the turning plate is provided with the initial position suitable for a large paper roll to be supported on the second arc groove and the upturning position suitable for a small paper roll to be supported on the first arc groove. When the printing paper rolls are nearly used up, the small paper roll is limit inside the first arc groove along with the rotation of the turning plate so as to prevent the small paper roll from shifting along a paper bin during printing. When printing paper surplus is smaller than the distance between the sensor and a printing mechanism, the sensor can send out a paper out signal timely and accurately, thereby improving the paper detection reliability of the sensor.