The present invention provides a battery module state detection method. The battery module includes a battery unit and is connected to a load. The method for detecting the battery module status includes the following steps. First, measure an on-load voltage value of the battery unit and a load current value of a load. Next, calculate an instant resistance value of the battery unit according to a no-load voltage value of the battery unit, the on-load voltage value and the load current value. Finally, obtain an operating state of the battery unit according to the instant resistance and a relationship between the internal resistance and discharging time. By the method of the present invention, the user can monitor the remaining energy of the battery module accurately in real time and therefore prevent the electrical hazard caused by over-discharging the battery module.