A novel fiber optic bending sensor based on LP21 mode is developed. A fiber affixed onto an elastic substrate is used to sense bending angles in the real time by measuring rotation of LP21 mode spot image. Theoretical analysis is given by fiber coupled-mode theory combining with elastic-optic theory. Relation between spot-rotation angle and sensor bending angle for different structures and temperatures is tested. The experimental results agree with theoretical analysis and validate the accuracy of the sensor. The sensor is temperature-immune and can detect bending direction as well as bending angles with a large dynamic range.