The invention provides an optimal dispatching method and system of a distributed photovoltaic cluster. The optimal dispatching method comprises the steps of calculating an optimal dispatching model, which is built in advance, by a master station to obtain a short-time scale optimal dispatching result, and issuing the short-time scale optimal dispatching result to a sub-station; receiving the short-time scale optimal dispatching result issued from the master station by the sub-station, and allocating the short-time scale optimal dispatching result to each photovoltaic field station; receiving an allocation result of the sub-station by the photovoltaic field station, and performing calculation to obtain a short-time scale dispatching instruction by combining the allocation result and each photovoltaic unit in the photovoltaic field station; and executing the short-time scale dispatching instruction by the photovoltaic unit to obtain an execution result. By the optimal dispatching method,the running cost and the circuit loss are reduced, and the distributed photovoltaic grid-connected capability and the control result are improved; and by the optimal dispatching method, the control dimensionality and the calculation complexity are effectively reduced, dispatching effectiveness and reliability are improved, a long-time scale optimal dispatching result is updated according to the short-time scale optimal dispatching result, and dispatching accuracy and effectiveness are improved.