The invention discloses an oil circulation and molecule exchange based transformer oil immersed gas removal apparatus and a method thereof. The apparatus adopts a molecule exchange principle of injected nitrogen in a laboratorial insulating oil oscillation degassing process and also adopts an oil circulation sprinkling mode to substitute an oscillation mode in order to improve the stability and the reliability of a system. A degassing mode in the invention completely stimulates the laboratorial degassing mode, so an analytic result is basically same with the analytic result of the laboratory of a power research institute only when a gas analysis method is accurate, and a transformer running state evaluating result will be consistent with the evaluating result of the laboratory of the power research institute, thereby it is helpful for improving the gas analysis based transformer running state online monitoring accuracy.