In order to obtain the optimal rolling and cooling processes-parameters for producing C-Mn-Si dual phase steel on CSP line, computer-aid modeling austenite decomposition during laminar cooling is carried out in present paper. Effects of chemical compositions, finishing rolling temperature (FT7), isothermal holing temperature (HT) and isothermal holding time (t) on austenite decomposition are investigated. The optimal steel chemistry and rolling and cooling processing-parameters are obtained through the modeling. According to the modeling results, 580MPa grade C-Mn-Si DP steel is successfully produced on CSP line with 0.05~0.07mass%C,1.20~1.50mass%Mn, 0.50~0.70mass%Si, FT=820~840°C, HT=680~720°C,t=8~11s. Comparison of modeling results and industrial trials show that current modeling structure is able to model and optimize processing parameters for C-Mn-Si DP steel production on CSP line.