The obvious transient absorption at the laser wavelength in Nd:YAG laser crystals is attributed to the production of transient colour centres, induced by UV light irradia-tion. The filtering characteristics of the materials are investigated by means of this phenomenon. The filtering materials suitable to lasers are selected effectively using the combined Nd: YAG cw laser system. In addition, we obtained the relation between pumping powers of different rods at the threshold in the combined laser. As compared with Mori's, our formula is a quantitative expression containing a variety of coefficients.