A data writing method for a flash memory and a control circuit and a storage system using the same are provided. The data writing method includes determining whether the size of data to be stored by a host system is smaller than a predetermined value according to a write command received from the host system, when the size of the data is smaller than the predetermined value, the data is written into a corresponding buffer physical block or a corresponding spare buffer physical block. The data writing method further includes combining valid data belonging to the same logical block during the executions of several write commands. Accordingly, the response time during the execution of each write command is shortened, and the problem of timeout is avoided.