The invention provides an economical monitoring device and method for adjustment of operation modes of a circulating water system of a power station. The device comprises a power measuring device, an absolute pressure measuring device, a data acquiring unit and a calculating and displaying unit. The power measuring device comprises a first power transmitter (1), a second power transmitter (2), a third power transmitter (3), a fourth power transmitter (4) and a fifth power transmitter (5), which are used for measuring power of a turbo-generator and power of four circulation water pump motors. The absolute pressure measuring device comprises a first absolute pressure transmitter (6) and a second pressure transmitter (7) which are used for measuring high/low pressure condenser pressure of a turbine. According to the invention, the monitoring device is used for acquiring onsite real-time data so as to calculate economic dispatch operation indexes of a circulation water system. Thus, the system is simple; measurement and calculation results are precise; actual energy conservation results before and after adjustment of operation modes of the circulation water system can be directly displayed; and requirements of economic operation of a power station are satisfied.