The present invention relates to a method for manufacturing a flat-plate battery. The method includes step S1: providing a chlorophyll layer; step S2: providing a first separator and a second separator absorbing a solution of organic salt respectively; step S3: providing a negative-electrode layer; step S4: coating the first separator on the negative-electrode layer; step S5: coating the chlorophyll layer thereon; step S6: coating the second separator thereon; step S7: coating a positive-electrode layer thereon; and step S8: sandwiching them between an upper plate and a lower plate. The flat-plate battery manufactured by the method can store hydrogen by the chlorophyll of the chlorophyll layer, and this method will not cause environmental pollution even when the flat-plate battery is discarded after use.