This paper presents the mechanical properties and welding properties of Q235 steel with minimum yield strength of 235 MPa treated by a novel quenching－partitioning－tempering (Q-P-T) process.The experiments indicate that the strengths of Q-P-T treated Q235 steel (briefly called QPT235 steel) markedly raise compared with Q235 steel, and its yield strength and tensile strength are 435 and 615 MPa, respectively. In addition, the mechanical properties of the welding joint of QPT235 steel are markedly improved compared with Q235 steel when the same welding solder and process are performed for the steel with the two treatments, and the tensile strength and elongation of the former are about 532 MPa and 16.71%, respectively, while those of the latter are about 414 MPa and 12.4%. The microstructural characterization reveals two main factors resulting in the mechanical properties of QPT235 steel superior to those of Q235 steel: the grains of ferrite and interlamellar spacing of pearlite are both refined in the welding heat affected zone (HAZ), and a lot of widmanstatten structures in the welding joint of Q235 steel is avoided for QPT235 steel; there is a mixed microstructure of hard phases of martensite and bainite as well as remained austenite as soft phase in both base metal and HAZ, which replace parts of ferrite and pearlite in Q235 steel.