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A large and sudden increase in radiocarbon ((14)C) around AD 773 are documented in coral skeletons from the South China Sea. The (14)C increased by ~ 15‰ during winter, and remain elevated for more than 4 months, then increased and dropped down within two months, forming a spike of 45‰ high in late spring, followed by two smaller spikes. The (14)C anomalies coincide with an historic comet collision with the Earth's atmosphere on 17 January AD 773. Comas are known to have percent-levels of nitrogen by weight, and are exposed to cosmic radiation in space. Hence they may be expected to contain highly elevated (14)C/(12)C ratios, as compared to the Earth's atmosphere. The significant input of (14)C by comets may have contributed to the fluctuation of (14)C in the atmosphere throughout the Earth's history, which should be considered carefully to better constrain the cosmic ray fluctuation.