A data storage system and a global deduplication method thereof are provided. The data storage system includes multiple storage devices and one dispatch device. The dispatch device divides an original data corresponding to a data writing request into at least one data chunk. The dispatch device performs a summary calculation on one data chunk, so as to generate a representative value. The dispatch device performs a first distribution calculation on the representative value, so as to determine a destination location corresponding to the representative value. The dispatch device transmits the data chunk and the representative value to at least one destination storage device among the storage devices through a communication network according to the destination location. The at least one destination storage device checks the representative value, so as to determine whether to store the data chunk in a storage space of the at least one destination storage device.