The energy scale during preheating after inflation can be very high, namely, about 1013GeV. In this thesis, we investigate the preheating theory by introducing higher- order coupling terms to extend the preheating theory after inflation to high energy scale and try to verify the robustness of the preheating theory. We find that the preheating theory is robust only for small coupling constants. Higher-order coupling terms might enhance or suppress the parametric resonance, and lead to the preheating more or less efficient.
