Abstract: Ultra-heavy plate steel for marine engineering usually requires good hardenability, and rational design of chemical composition of this steel is one of the main methods for improving its hardenability. Traditional composition design of steel is mostly experiment-based and costly. In this paper, thermodynamic calculation software Thermo-Calc and material property calculation software JMatPro were introduced to design the chemical composition of B-containing micro-alloyed ultra-heavy plate steel, with calculation, prediction and testing combined to obtain high hardenability. Thermodynamic equilibrium precipitates in the steel were calculated with Thermo-Calc. Composition design principles were interpreted for the steel from the perspective of thermodynamics by analyzing the precipitation temperature; precipitation amount and phase relationships, and then the chemical composition of the steel with high hardenability was designed. In addition, the hardenability of the steel designed was predicted using JMatPro and the results illustrates that the composition design of the steel is feasible. The rationality of the optimization design was further affirmed by hardenability testing and chemical analysis. Theoretical calculation and experimental results show that high aluminum content can help to improve the hardenability of the steel.