The design and fabrication of offshore structure depend on material science and solid mechanics, such as steel processing technology and welding and materials testing and evaluation and so on. In this paper, CTOD (Crack Tip Opening Displacement) toughness test for weld joint of heavy thickness S355G10+N steel has been carried out to evaluate the fracture resistance of this weld joint under Submerged Arc Welding(SAW). The results show that (1) the weld joint of heavy thickness S355G10+N steel tested possesses excellent toughness. That is to say the materials in the weld joint possess enough capacity resisting the crack and propagation. Therefore the welding procedure specification evaluated can be applied to fabricate offshore structure. What is more, being at the case of as-weld condition for the weld joint the CTOD test was carried out herein, hence the offshore structure fabricated with the welding procedure specification evaluated can avoid the post welding heat treatment. (2) Different positions in weld joint have their special toughness. The position which is 2mm away from the fusion line (located in base metal) (FL+2) possesses best toughness (average value is 0.825mm). The fusion line position (FL) possesses slightly worse toughness (average value is 0.672mm). The weld metal position possesses worst toughness (average value is 0.553mm). This distribution character is different from other references reported and comparision has been made in this study. (3) Being a lower fracture toughness in weld metal, as a design suggestion high stresses should be avoided in this position.