Method and system of measuring blood circulation velocity

A system and method of measuring blood circulation velocity includes the steps of keeping a person measured by a blood oxygen saturation analytical instrument and served by a ventilator; setting an initial time point, providing a first gas to the person via the ventilator, and starting to record a blood oxygen saturation value per predict time interval; providing a second gas to the person at a first time point; providing a third gas to the person at a second time point; stopping recording the blood oxygen saturation value at a terminal time point; setting a reference time point according to the blood oxygen saturation value which has a variation according to the records; obtaining the person's blood circulation velocity, which is proportional to the reciprocal of the difference of the reference time point and the first time point..