A rigid fiber-optic imaging element with high fidelity, high resolution, and high contrast is applied in low-level-light night vision and particle detection devices. Any optical fiber in elements is an image transmission unit. However, the independence and integrity of image transmission are disturbed constantly, resulting in blemishes. This paper studies the formation mechanism of the blemishes of rigid fiber-optic imaging elements. The existence of defects with different sizes in or among the optical fibers, and fiber deformation decreasing light transmission ability are simulated theoretically. Then experiments are carried out to verify the simulation results. It is theoretically concluded that a blemish forms when the equivalent sphere radius (RES) of a stone defect is greater than 1.2 µm in a normal fiber, and 1.1 µm in a taper fiber. The RES of a bubble defect is greater than 1.3 µm in the fiber, which can form blemishes. Excessive deformation of fibers behaving as a clad layer with thickness less than 0.3 µm also results in the formation of blemishes. When fiber deformation and the existence of stone are considered, blemishes can occur as long as the size of the stone defect among the fibers is greater than 0.823 µm. The results provide data support and theoretical guidance in solving blemishes and other fixed pattern noises, and significantly improve the engineering and application level of fiber-optic imaging elements.