The invention provides a preparation method of a water system sodium ion battery cathode material and a water system sodium ion battery. The method comprises the following steps of: taking a sodium source and a manganese source in a preset proportion, adding a solvent into the sodium source and the manganese source, performing uniform mixing and drying to obtain mixed powder; and fully grinding the mixed powder, and performing calcining at 600-900 DEG C for a period of time to obtain a cathode active material. According to the invention, the sodium source and the manganese source are fully mixed, then fully ground and calcined at a certain temperature to prepare the cathode material, the raw materials are low in price, the operation method is simple, the reaction conditions are mild, the requirement on equipment is low, the preparation cost of the water system sodium ion battery is favorably reduced, the charge-discharge specific capacity of the prepared cathode material is up to 85 mAh/g, the cycling stability is good, and the overall electrochemical performance is high.