In Space Solar Telescope, the decentering errors and air spaces between five components of collimating lenses are so small that subcell assembly method has to be adopted. This paper introduces a new method that can easily center lens with three small steel balls and some normal devices. The thought comes from traditional edging technology of lenses. When calibrating a lens, three precise steel balls will give the basis of positioning, After the optical axis of the lens is consistent with inner axis of subcell, the lens and the subcell are bonded together with some RTV adhesive. With all five subcells bonded with those corresponding lenses, they are assembled together again based on the outer reference surfaces that have been machined before. The precision of this method depends on the quality of steel balls and the parallelism of subcells. With the help of ZEMAX calculation, a collimating telescope is used to test the calibration error of the lenses. And steel balls are also introduced to quickly measure parallelism of subcells, and one interference graph of testing results is also presented.