The microstructure, properties and strengthening mechanism of 500 MPa grade NbTi microalloyed square pipe steel were investigated by utilizing the optical microscope (OM) and transmission electron microscopy (TEM). The results showed that the finish rolling temperature (FDT) and the coiling temperature (CT) had considerable influences on the microstructure and mechanical strength. The decrease of the FDT and CT was benefitted to obtain the finer ferrite grain size and finer dispersion of the second phase precipitate in the studied temperature range. The yield strength, tensile strength and elongation rate were increased with the decrease of the FDT when the CT was constant. The yield strength and tensile strength displayed the tendency of declining at the beginning and rising up in late, and the elongation rate was monotonically increased as the CT decreased gradually. The optimal mechanical properties were obtained when the FDT and CT were 840 and 570 ℃, respectively, and the yield strength, tensile strength and elongation rate were 537 MPa, 578 MPa and 33.5%. The grain refinement strengthening was the most important strengthening mode, which accounted for 49% to 51% of the yield strength, and the solid solution strengthening was the second main strengthening mode, which accounted for 23% to 27%, while the precipitation strengthening was only accounted for 3.8% to 8.2% of the yield strength.