Studying major depressive disorders and related factors is necessary to understand the nature of this disorder and to formulate more effective therapeutic strategies. Approximate duration of one episode of major depressive disorder is averagely between 3 to 12 months; however, in chronic types, the ratio would be prolonged to 24 months. The aime of this research which performed through second-order dynamic model was to investigate the effectiveness of antidepressants and third wave cognitive-behavioral therapies on the duration of any episode of major depressive disorder. Through MATLAB programming and changing the model parameters, we investigate the effectiveness of treatments on duration of episode in major depressive disorder. The average age of any participants were considered  64 years old. Outcomes before and after 14 weeks treatment with antidepressants drugs (as imipramine or its equivalent of therapeutic services) and psychotherapies (included mindfulness-based acceptance and commitment therapty [ACT] and  dialectical behavior therapy [DBT]) were obtained. According to the findings, for most of people suffering from major depressive disorder, relapses are prevalent phenomena. Depression relapse time durations have been 144, 71, 68, and 59 months respectively under following conditions: receiving no treatment; receiving untidepressants; receiving dialectical behavior therapy; and, receiving a combination of antidepressants and dialectical behavior therapy. The results from dynamic modeling showed that combined method of ACT, DBT, and using antidepressant (or its equivalant) drugs provides better outcomes in treatment for depression and relapse prevention. This combined method reduced the duration of recovery from depression by 10 months.