The invention introduces a method for atomic writes in an SSD (Solid State Disk) system, performed by a processing unit, including at least the following steps. An atomic-write command instructing the processing unit to write first data whose length is less than a page length in a storage unit is received. When it is determined that the atomic-write command will trigger a cross-page buffering of a buffer, dummy data is filled into all available sub-regions of a first region of the buffer, and the first data is stored in a second region of the buffer. After the first data is successfully stored in the second region of the buffer, a safe pointer is modified to point to the end address of the last sub-region of the second region, which stores the first data.