Invasive microwave coagulation (IMC) therapy is a newly developed method for the treatment of liver tumors. In this paper, the constant-power IMC therapy is discussed. Three-dimensional finite element analysis for "alternative" heating pattern and a large blood vessel bioheat model are performed. The objective is to study: (i) the effects of "alternative" heating pattern of invasive microwave antennas on reducing the peak temperature which can be up to 140℃. Therefore, "alternative" heating pattern can improve the security of IMC; (ii) modeling impact of large vessels on heat transfer of surrounding tissue and the formation of the coagulation lesion. Corresponding in vivo pig experiments are provided to compare with the simulation results. The results indicate that: (i) the "alternative" heating pattern was efficient in reducing the peak temperature and diminishing the dimension of "super hot" area; (ii) the heat loss due to the cooling of the large blood vessel is so great that attempting to compensate the cool area by thermal method is unfeasible, which explains why occluding the blood vessel is a practical approach to dealing with this problem during the invasive microwave coagulation therapy; (iii) the radius of effect of large vessels (Rev) was brought forward to provide theoretical basis for clinical practice.