An analytical formula is derived from elementary expression of molecular total energy for calculating the accurate P-branch spectral lines of rovibrational transitions for diatomic molecules. Using the constants (Bυ',Bυ″) and a group of fifteen known P-branch experimental transition lines for a transition band, the formula not only reproduces the known experimental transition lines, but also generates the spectral lines that may not be available experimentally. The P-branch emission spectra of the (2,0) band of the CO molecule are studied in this work, and correct values of the unknown spectral lines up to J = 80 for this band are predicted by using the formula.