The invention discloses a casting powder-magnesium grain cored wire, application and a production process of steel used for high heat input welding. The cored wire comprises an outer layer and a core. The outer layer is made of low-carbon steel and is 0.5-1.0 mm thick. The core is made of slow-release passivated magnesium grains and comprises a slow-release agent and the passivated magnesium grains. The slow-release agent is continuous casting powder and accounts for 5-90% of the content of the core; and the passivated magnesium grains account for 10-95% of the content of the core. The cored wire is applied to preparation of the steel used for high heat input welding. According to the casting powder-magnesium grain cored wire, application and the production process of the steel used for high heat input welding, the slow-release passivated magnesium grain cored wire with the continuous casting powder as the slow-release agent is fed into liquid steel, the Mg absorption rate is stable, Mg plays a big role, and the utilization ratio of Mg is high; and inclusions are dispersed and rich, and the inclusions containing Mg reaches 80% or above. By adoption of the casting powder-magnesium grain cored wire, application and the production process of the steel used for high heat input welding, control is easy, the production cost is low, and the steel used for high heat input welding can be produced industrially in a batched mode.