A biodegradable ceramic fiber composition for a high-temperature thermal insulator is provided. The composition includes: 58 to 67% by weight SiO2, 26 to 34% by weight CaO, 2 to 8% by weight MgO, 0 to 1% by weight Al2O3, 0 to 5% by weight B2O3, 0 to 3% by weight Na2O+K2O, and 1% by weight or less impurities selected from TiO2 and Fe2O3. The composition has a linear thermal contraction coefficient of 3% or less (when maintained at 1100° C. for 24 hours) and a dissolution rate constant of 700 ng/cm2·hr or more in a synthetic body fluid. When compared to known biodegradable ceramic fibers, the ceramic fiber composition also has a significantly improved solubility in a synthetic body fluid so that it can easily be dissolved and removed even when inhaled into the human lungs, thereby reducing harmfulness to the human body.