Schedule power flow calculation is an important part of the security constrained economic dispatch (SCED). General foreign net equivalent is rough and the results are of large deviations, therefore an improved method of active power flow adjustment in security constrained economic dispatch is proposed. First, the schedule data and the network model are acquired to make power generation plan without security constrained. Then schedule power flow is calculated under consideration of sensitivity with quasi-steady state, tie line adaptive equivalence and active power balance. Making new power generation plan with security constrained is needed when active power of line is off-limit. The method is simple and can improve the accuracy of schedule power flow and provide credible information for power generation plan. Test results on the practical engineering examples show that the method can meet the practical requirements, with good reliability and practicality.