TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The aquatic plant comprises microalgae. Microwave cracking is conducted at 135-700, preferably 400-600 degrees C for 2-15 minutes using microwave power of 500-60000, preferably 1000-10000 W. The product cracked by microwave is further separated. The generated solid is discharged and collected directly. The high temperature gas generated by cracking is cooled by condensing, where condensed part is flowed out of the collected liquid as biological oil, and the non-condensed part is separated, dried, and collected as gas i.e. firing gas. The separated solid is screened, where refined powder is used for preparing organic fertilizer and rough powder enters the microwave reactor for secondary cracking. The method uses special device, which comprises screw rod propeller, feeding hopper, microwave generator, microwave reactor, slag storage pot, condenser, oil storage pot, water pump, cooler, cooling water pot, gas washing pot, drier, gas storage pot, fuel generator, drying system, and roots fan. It comprises putting aquatic plant into drying system to dry, controlling the water content at less than 15%, feeding the aquatic plant to the feeding hopper by using screw rod propeller, and feeding to the microwave reactor by propelling via the screw rod or feeding the dried aquatic plant to the microwave reactor by vacuum absorbing; microwave cracking at 135-700 degrees C for 2-15 minutes using microwave power of 500-60000 W; separating the solid and connecting the slag storage pot via a lower outlet of the microwave reactor to collect the solid; separating gas and solid by cooling the high temperature gas generated by hot cracking in microwave reactor via a three section condenser, flowing the condensed part to the oil storage pot to collect the biological oil, flowing the non-condensed gas to gas washing pot, drying by drier, flowing into gas storage port to collect as fuel gas, and generating by fuel gas generator or firing directly to provide heat for the drying system; feeding cold water to the condenser from the cold water pot via water pump, and flowing back to the cold water pot by the returned condensed water via the cooler; maintaining a negative pressure state by using roots fan, controlling the pressure at - 0.005-0.04 MPa, and preventing air from entering the system; and recycling and circulating the generated heat for the drying system.