Based on the Pennes bioheat transfer equation, location and flow rate of a large blood vessel effect on microwave thermal ablation was analyzed, in which the convection heat transfer of the cooling water was involved. Numerical results showed when the distance between the antenna and a blood vessel exceeded 20mm; the ablation area was elliptical-like. The blood vessel effect on the ablation can be ignored. Doctor needn't operate the blood vessel blockade before microwave thermal therapy. But when distance was less than 10mm, the other side of blood vessel also has been coagulated. Flow rate effect on the ablation area is small.