Abstract: The development of a high-precision model for forecasting the medium- and long-term power availability of photovoltaic (PV) power is critical for the development of monthly plans in the electricity market dispatch model. Therefore, a statistical model for forecasting the PV power generation availability based on a differential autoregressive moving average model is developed to realize a horizontal monthly-moving forecast on the future inter-annual basis. Then, the monthly variability of light resources and the smoothness of the same season are taken into account, and a correction method for forecasting the PV power generation availability based on the hidden Markov model is proposed to achieve a vertical monthly forecast correction with the same monthly recurrence of variability. The validity and accuracy of the method are confirmed by using PV operation data from a Xinjiang power grid region.