Abstract: Since the cost is rather high for directly using the steel pipe to conduct the testing of the performance of the welded joint，usually the small test plate is used instead of the steel pipe. However，with the same welding parameters，there is a significant difference in the impact toughness between the steel pipe and the test plate. Accordingly the thermal conductive silicone is added at the bottom of the test plate so as to get the result similar to the impact toughness of the steel pipe，which indicates that the temperature field is an important factor as leading to the impact toughness difference between the test plate and the test steel pipe. Owing to jointly using the finite element simulation software with the welding process experiment，a numerical analysis model for the steel test plate and the steel pipe of the X65 pipeline steel is established. Then the temperature fields of the welding test plate and the test pipe are simulated，and calculated. Furthermore the reasons for the difference in impact toughness between the said test pipe and the test plate are analyzed by comparing the joint microstructure and the hardness distribution of the two. And the specific difference in cooling time between the two is quantified，providing a theoretical basis for the follow-up silicone addition to adjust the temperature field data.