The invention discloses a method and a system for synchronously updating online strategy definite values. The method comprises the following steps: when the time interval of every two adjacent onlinestrategy definite value calculations of safety control systems is greater than a critical value at a dispatching end, calculating the online strategy definite values corresponding to the safety control systems at the dispatching end, and sending definite value generating moment T0, invalid moment Tt and respectively corresponding online strategy definite value lists to the safety control systems;determining that respective online strategy definite values are received by each buffer area of the safety control systems at the dispatching end, determining that the online strategy definite valuesof the safety control systems meet validity checking respectively, and when the absolute time t of the safety control systems is in advance of invalid moment Tt, controlling each safety control systemto be completely updated into checked online strategy definite values after handover delay delta T is met. According to the method disclosed by the invention, time marks are taken as references, thereliability of the online definite values is checked, and the synchronism for the online definite values of the large-scale safety control system is also guaranteed.