Abstract: Welding is an important process in the manufacturing of marine equipment, and the performance of welded joints directly affects the safety and reliability of the equipment. In response to the development needs of high performance marine materials, this study designed and prepared a 460 MPa grade high-strength offshore engineering steel with low P_cm. Welding thermal simulation tests were carried out using an MMS-300 thermal simulator to reveal the toughening and embrittlement mechanisms of the heat-affected zone and to optimize the process parameters. The effects of welding heat input, preheating temperature, and peak temperature on the microstructure and toughness of the heat affected zone were analyzed using Charpy impact tests, optical microscopy, and scanning electron microscopy. The fracture characteristics of the coarse grained zone and the intercritical coarse grained zone under single pass and double pass thermal cycle conditions were compared. The results show that with increasing welding heat input and preheating temperature, the microstructure of the coarse grained zone gradually transforms from lath bainite to granular bainite. The content and size of M/A constituents increase, and the impact toughness continuously decreases. The fine grained zone achieves grain refinement through complete austenitization and recrystallization, exhibiting the best toughness. The intercritical zone shows heterogeneous microstructure due to partial austenitization, and its toughness is lower than that of the fine grained zone. The intercritical coarse grained zone exhibits microstructure heredity after experiencing double pass thermal cycling. Austenite inherits the coarse microstructure from the coarse grained zone, and chain like M/A constituents form along the prior austenite grain boundaries. This not only widens the grain boundaries but also acts as crack initiation sites, further deteriorating the impact toughness, making it the weakest region in the welded joint. This study provides a reference for the development of thick plates of weldable marine engineering steel, the optimization of welding processes, and the quality control of welded joints.