The invention discloses a vacuum design method of a sealed cesium-beam tube. An ideal vacuum degree or pressure can be obtained with a low cost, and the development success rate of the cesium-beam tube is increased. Firstly, the range of the work pressure of the cesium-beam tube is obtained according to a relation curve of the cesium-beam intensity and the pressure inside the cesium-beam tube, then according to the still standing time requirement of the cesium-beam tube, the air output of the material unit time and average air output rate of the material unit area when all components of the cesium-beam tube are subjected to still standing are obtained, through the combination of the air output in the unit time of components of a cesium furnace and the air output in the unit time of components of a detector after electrification, the total air output of the cesium-beam tube in the unit time is obtained, then according to the total air output in the unit time and the range of the work pressure of the cesium-beam tube, the minimal pumping speed of a sputter ion pump is calculated, the structure of the sputter ion pump is determined, and finally a degassing technology of the cesium-beam tube is formulated according to the average air output rate of the unit area of all materials of the cesium-beam tube. The vacuum degree of the cesium-beam tube obtained thereby meets a design requirement, and the cost is low.