Abstract: The hardenability of steel is commonly said of the Jominy test curve. Drawing curve by hand, error is big; build the curve equation is difficult and the figure is not convenient to communicate. The Jominy test folding line with equation were introduced. The Jominy test folding line with equation besides has hardenability curve function, can also be used to estimate half martensite hardness and the distance of critical diameter from quenching end. The distance of critical diameter can be used as a measure parameter of hardenability of steel. Drawing floding line is simple and easy to build the equation. The method of linear spline function to construct equation was introduced. The Jominy test folding lines and equations for steel 40CrH, 40MnBH, 60Si2Mn and 20CrMnMoH were also introduced.