ABSTRACT Objective To analyze the influence of different blood vessel diameters on the heat deposition in the liver para-vascular microwave ablation under ultrasound guidance.Methods The PP plastic pipes simulated blood vessel pig liver model (experimental group) and the pig liver model without simulated blood vessel (control group) were established respectively. The experimental group was divided into 5 groups according to the simulated blood vessel inner diameter, namely group A (inner diameter of 4 mm), group B (inner diameter of 5 mm), group C (inner diameter of 6 mm), group D (inner diameter of 7 mm) and group E (inner diameter of 8 mm). The parameters of ablation lesions in each group were observed and compared, including maximum radius (Rmax), ablation area (Sz), and area change rate (Sdiff).Results Under the same ablation power and time, there were significant differences in Rmax and Sz between the experimental groups and the control group (all P < 0.001). There was no significant difference in Rmax, Sz and Sdiff between group C and group D, and between group D and group E in the experimental group. There were significant differences in Rmax, Sz and Sdiff between any two groups (all P < 0.05); The inner diameter of the simulated blood vessel in the experimental group was different by 2mm or more, and the differences in Rmax, Sz and Sdiff within the group were statistically significant (P < 0.05).Conclusion When the distance between the ablation needle and the simulated blood vessel was constant and the inner diameter of the simulated blood vessel was less than 6mm, the heat deposition effect increased with the increase of the inner diameter of the simulated blood vessel. When the diameter of the simulated vessel was larger than 4mm, the reduction rate of the maximum cross-sectional area of the ablation focus was more than 30%.