Q345R steel is the most commonly used material in fabrication of the pressure vessels and boilers in China, due to its excellent properties. In 2010, ASME code case 2642 accepted Q345R steel for use in construction of pressure vessels. The code case specified impact test exemption curve A for the impact test requirements for Q345R. However, this provision severely limits the application of this material at low temperature, since most of the minimum design metal temperature (MDMT) of curve A is above the freezing point. In this paper, a series of tests (such as uniaxial tensile test, impact test, and fracture toughness test) were carried out at low temperature to investigate the mechanical properties of Q345R steel plates with thickness of 36–80 mm. This study of low temperature usage of Q345R steel was conducted using the fracture mechanics assessment procedure of API 579-1/ASME FFS-1. The fracture toughness is given by master curve (MC) method in the transition regime. The results show that Q345R can be used at lower temperature and that classifying Q345R steel into curve D is appropriate.