An analytical formula is derived from conventional expression of transition spectra containing higher terms rovibrational spectral constants Lv based on the difference converging method (DCM), which can calculate the accurate P-branch spectral lines of rovibrational transitions in some diatomic molecules systems. Based on the DCM method, a group of eleven known experimental transition lines is needed and a set of expansion coefficients C is chosen to predict correct transitional spectra of high-lying rovibrational states for a P-branch transition band of a diatomic system. In this study, the transitional spectral lines of high-lying excited states which may not be available experimentally and the rotational spectral constants of the corresponding transition bands were obtained correctly by using the DCM method for the (3, 7) band of A2Πu-X2Πg(Ω=1/2) transiton of the 35Cl+2 and 35Cl37Cl+2 ions. The calculated data can provide a better reference for the further understanding of the internal structure and physical and chemical properties of electronic states of the Cl+2 ion.