The invention provides an online simulation system and method for a power plant. The system comprises a simulation control module, an online simulation application module, a data obtaining module and an online simulation database. The data obtaining module is used for obtaining production real-time data and production historical data and transmitting the production real-time data and the production historical data to the online simulation database; the online simulation application module is used for calling a simulation module function, the production real-time data and the production historical data from the online simulation database for simulation analysis, and writing a simulation result into the online simulation database. By means of the system, the production real-time data and the production historical data are obtained through the data obtaining module and are transmitted to the online simulation database, so that the relevance degree of data sources of the online simulation database and the onsite real-time production data is large, and simulation analysis precision is high.