The invention provides an online monitoring method and monitoring system for margin improvement of nuclear power unit thermal power, wherein the monitoring method comprises the steps: building a real-time database, collecting real-time data of unit output related characteristic parameters, and storing the real-time data in the real-time database; calculating the difference value, the maximum value, the mean value, the sliding mean value and the overtime duration of the characteristic parameters according to the real-time database; comparing the real-time data and the calculated values of the characteristic parameters with corresponding alarm limit values, and judging whether the real-time data and the calculated values exceed the corresponding alarm limit values or not; and if the real-time data and the calculated values exceed the corresponding alarm limit values, triggering an early warning function to give an alarm. The real-time data of the characteristic parameters and the difference value, the maximum value, the mean value, the moving average value and the overtime duration of the characteristic parameters are used as thermal power control parameters and change of the thermalpower control parameters is tracked in real time; real-time online monitoring of the margin improvement of the nuclear power unit thermal power is achieved, the unit output is improved, and maximization of the power generation capacity of a power plant is achieved on the premise that safe operation of the unit is guaranteed.