According to the statistics of planetary period ratios in a multiple planetary system, the excesses appear just over the exact integral ratios, and the deficits on the near left side. There are various dynamical explanations for the phenomenon. For the three observed planets in Kepler-9 system, the period ratio between the planet b and c approximately equals 2.03, which is a typical sample close to the 2:1 resonance. For the two conditions of considering secular interactions only and adding resonance perturbations in, we applied the Hamiltonian functions with the second-order eccentricity terms to provide relative locations of current orbital configurations in the level curves of energy and cross section, then discussed the probable near resonance states of the two planets.