Instantaneous gas water heater is designed specially for RV hot water system, and the whole operating process is controlled by its microcomputer intelligent control system, to ensure continuous supply of constant temperature water. The multi-safety devices such as overheating, anti-freezing, flame out, dry-burning, water overpressure protection, are monitored by the microcomputer intelligent control system so that the heater will be shut down immediately in case of any malfunction to ensure the safety.