A battery module and an overcharge protecting method are provided. The overcharge protecting method includes following steps: detecting a battery voltage of a rechargeable battery during a charging process; determining whether the battery voltage is larger than or equals to an upper limit voltage; if no, continuously charging the rechargeable battery until the battery voltage reaches a charging cut-off voltage and then the charging process is ended; if yes, stopping the charging process, reducing the value of the charging cut-off voltage and using the reduced charging cut-off voltage as the charging cut-off voltage in the next charging process. In the subsequent charging process, once the overcharge occurs, the value of the charging cut-off voltage is continuously reduced until the overcharge of the rechargeable battery does not occur during the charging process.