The thermal efficiency of the fast gas water heaters is an important index sign to reflect the efficiency of its utilization for energy resources,the executing of the signs and standers of utilization for energy resources of gas equipments will put forward some very high request with the objectivity to it. This index sign is tested and defined under the single experimental condition.But that is different in practice.The water heaters work under the various conditions and the changes of the thermal efficiency parameters are inevitable.Therefore the thermal efficiency evaluation for the fast gas water heaters can not carry through with the thermal efficiency which is tested under the single condition and the all-year average thermal efficiency should be adopted.Aiming at the orderliness about the changeful thermal efficiency of the fast gas water heaters which work under the different conditions,we do a flood of experiment research and scientific theoretic analysis ,and bring forward a new evaluation means for the thermal efficiency,that is the all-year average thermal efficiency should be adopted in evaluation.This paper also introduces the calculational methods of the all-year average thermal efficiency detailedly.