A methodology for studying the unconfined aquifer drainage progress through the drainage experiment of the cylindrical sand is introduced. And it is used to discuss the variation of the specific yield with the depth to the water table and time during the experiment. The experiment result shows that the specific yield increases with the depth to the water table and the drainage time increasing. When the drainage experiment starts from the depth to the water table of 0 cm and ends at the stable water level of 15 cm, the maximum specific yield is 0.05. When the drainage experiment starts from the depth to the water table of 75 cm and ends at the stable water level of 90 cm, the maximum specific yield is 0.25. This study also shows that the value of the specific yield gets from the Nachabe empirical formula is more accurate when the depth to the water table is equal to or greater than 75 cm.