A method for programming a non-volatile memory and a memory system are provided. Each of multiple cells of the non-volatile memory stores data having at least 2 bits. The method includes the following steps. At least one programming pulse is provided for programming a target cell of the cells. At least one program-verify pulse is provided for verifying whether the target cell is successfully programmed. It is determined that whether a threshold voltage of the target cell is greater than or equal to a program-verify voltage. When the threshold voltage is greater than or equal to the program-verify voltage, the target cell is set as successfully programmed. Next, a post-verifying operation is performed to the successfully programmed cell. The post-verifying operation includes determining whether the threshold voltage of the target cell is greater than or equal to a post-verifying voltage.