At present, the highly active antiretroviral treatment is recognized as the most effective therapy to treat and control HIV/AIDS. Furthermore, it is still very concerned that why combination drugs is better than the single drugs. In recent decades, there are already some researches about the multiple combinatorial perturbation theory. However, these studies mainly rely heavily on the dynamics of transient characteristics. In this paper, we propose a method which only rely on the steady state, i.e. the combinatorial perturbation research based on the theory of the branch to depict why drug combinations is better than single drug treatment. In this work, we first proposed a basic mathematical model of HIV infection without drug resistance to verify that the monotherapy with reverse transcriptase inhibitors could present rapid suppression of HIV replication in the short-term. Furthermore, we modified the basic model by taking into the immune response mechanism and drug resistance. Using a bifurcation-based method of combinatorial perturbation we analyze the synergistic regulation action of combination drugs on treatment of HIV infection. As a result, our analysis provided some theoretical supports of combination drugs therapy for AIDS from the point of view of dynamics theory based on bifurcation.