We study the competition between the Kondo effect and the exchange interaction in the parallel double quantum dot (DQD) system within an effective action field theory. The strong on-site Coulomb interactions in DQDs are treated by using the Hubbard–Stratonovich transformation and the introduction of scalar potential fields. We show that a self-consistent perturbation approach, which takes into account the statistical properties of the potential fields acting on electrons in DQDs, describes well the crossover from the Kondo regime to the spin-singlet state in this system. The linear conductance and the intradot/interdot spin excitation spectra of this system are obtained.