A data storage system and a data storing method for the data storage system are provided. The data storage system includes a host unit, a storage unit, and a first input/output bus functioning as an interface between the host unit and the storage unit. The storage unit includes a non-volatile memory buffer unit and a flash memory unit. The non-volatile memory buffer unit includes a plurality of buffers arranged in parallel. The flash memory unit includes a plurality of data storage devices arranged in parallel to input and output data using a parallel method. In the method, a writing request is first classified into one of a plurality of grades according to a writing request frequency when there is a writing request and the writing requested data is stored in one of the non-volatile memory buffer unit and the flash memory unit according to the writing request frequency.