A medium-and-long-term power load measuring and calculating system comprises a storage module, an obtaining module, an input module, a load characteristic influential factor correlation degree calculation model, a screening influential factor predicted value calculating module and a medium-and-long-term load predicting and calculating module; on the basis of an improved gray correlation analysis model, the obtaining module, the input module, the load characteristic influential factor correlation degree calculation model, the screening influential factor predicted value calculating module and the medium-and-long-term load predicting and calculating module analyze correlation degrees between load characteristic index and all influential factors and obtain an influential factor sequence corresponding to each load characteristic index by screening, calculate predicted values of the influential factors by using a gray Verhulst model, and obtain principles by establishing a regression equation that satisfies the characteristic indexes and corresponding influential factors via a PLS model; and the modules are combined to calculate the predicted values of the load characteristic indexes, namely a medium-and-long-term load predicting result. The invention also provides a medium-and-long-term load measuring and calculating method.