Transient stability constrained optimal power flow is a complex nonlinear optimization problem. In the interior point method solution based on time domain simulation， transient differential equations are generally discretized and considered as additional equality constraints， which will easily suffer from the curse of dimensionality. Aiming at the deficiencies， an order-reduced interior point algorithm is developed. Taking the truncation error of specific discretization method into account， the proposed algorithm discretizes the transient equations into difference precision related inequality constraints and greatly reduces the order of correction equations in interior point method. Test results on several cases indicate that the proposed algorithm consumes much less time and memory compared with conventional approach， and it has the potential to deal with larger power systems.