Abstract: A main stream temperature control system for a large boiler comprises a PID module, A/D converters (M2, M3, M4, and M5), D/A converters (M6 and M7), electrical water spray adjusting valves (AA101 and AA102) and main stream temperature sensors (T1, T2, T3, and T4) for the boiler. The system uses a function module (f (x)), a differential module, a division module, multiplication modules (F1 and F2), addition and subtraction modules (J1, J2, J3, and J4), set value modules (K1-K6), selection modules (N1 and N2), a time pulse module (S1) and small value comparison modules (Z1 and Z2) in a distributed control system, so as to construct a real-time online optimized circuit and form an independent automatic control system for dynamic tracking and stable control, thereby increasing the thermal economic index of the boiler, and achieving the objective of energy saving and emission reduction.