Following the method of the adjusting coefficient k we introduced and using the simple analytical wavefunction within the weakest bound electron potential model (WBEPM), the transition results of each member in the isoelectronic series only vary with its nucleus charge number Z. So we can conveniently calculate the transition probabilities of Li I and the oscil-lator strengths of the 3-electron series. The comparisons with other methods show that not only the regularity of transitions in the isoelectronic series is reflected, but also a more accu-rate result could be obtained. The WBEPM theory with adjusting coefficient k can provide good trial wavefunctions for detailed atomic calculations, particularly for the study of isoelectronic series.