Abstract: In order to use the design conception of bubble-lift pump to enhance the natural-circulation ability of lead-bismuth alloy non-isothermal circuit, the filing loop bubble size needs to be chosen. The initial diameter of bubble has a marked impact on enhancing the natural-circulation ability for bubble-lift pump. In this paper, the VOF model was used to do a series of numerical simulations on the lifting effect of helium bubbles with different initial diameters in liquid lead-bismuth alloy. For the single bubble, the lifting ability increases firstly and then has an approximate constant with the increase of initial diameter. The bubble with a bigger initial diameter is easy to split into small bubbles which attach on the tube wall, and it might potentially affect the heat transfer efficiency. The rising velocity and rising height of helium bubbles with different initial diameters versus time in the static liquid lead-bismuth alloy and the cause of bubble division were studied.