This paper traces back to the popular saying that the utilization rate of river water resources for ecological protection should not exceed 40%. Tennant came to a conclusion that the river ecological condition was good when the river section runoff maintained at more than 60% of the annual average runoff based on the observation data of the corresponding relationship between runoff retention rate and river ecology of hundreds of small rivers in North America. Therefore, some scholars infer that the utilization rate of water resources should not exceed 40% under the condition that the returned water cannot return at the river section of water withdrawal. This is a statistical conclusion of large numbers. The threshold value of 40% should not be applied to specific rivers simply, but specific problems should be analyzed.