As oil and gas pipelines develop toward large throughput and high pressure, more and more attention has been paid to welding quality of oil pipelines. Submerged arc welding is widely applied in manufacturing of large-diameter welded pipes, and the welding quality has an impact on pipeline safety. With a multiwire submerged arc welding test platform and real-time temperature measurement system, temperature measurement has been done for multiwire submerged arc welding process with and without flux coverage, respectively. As a result, thermal cycling curves in both cases have been obtained, and convection and radiation coefficients of flux-covered X80 pipeline steel in air-cooled environment have been corrected. By using sysweld software, a finite-element computational model was set up for microstructure and residual stress in the weld zone of multiwire longitudinal submerged arc welding. Comparative experiment has been done to obtain welding temperature field with relatively high accuracy. Calculation and analysis of residual stress versus preheat residual stress decreased with increasing preheat temperature up to 100 °C, meanwhile content of bainite in microstructure fell, facilitating reduction in residual stress to some extent. This study provides quantitative reference for further optimization of welding parameters and improvement in weld mechanical properties.