The invention relates to a method and a system for online monitoring of loss of a power transformer, the method comprises the steps of: constructing a virtual current source in parallel with an inputend of a two-port network equivalent to a transformer by utilizing a current transformer, calculating a product of currents on a primary side of the transformer and a different current signal betweencurrents on a secondary side of the current transformer and currents on the primary side of the transformer, and take it as an output power of the virtual current source; constructing a virtual voltage source in parallel with an output end of the two-port network equivalent to the transformer by utilizing a current transformer, calculating the product of current on the primary side of the transformer and different voltage signal between voltage on the secondary side of the current transformer and voltage on the primary side of the transformer, and taking it as an output power of the virtual voltage source; calculating loss of the transformer by using power of the virtual current source and the power of the virtual voltage source. The invention provides a method instruction for measurementof loss of the power transformer under actual working conditions, the limitation of the measurement of loss of the power transformer is broken, and the effectiveness of the measurement of loss of thepower transformer is ensured.