Through the investigation of the actual operating data of northwest power system, this paper comes up with the depth of peak load regulation of combined operation of conventional thermal power generating units and heat supply units, and the principal factors that affect the regulation capability of hydropower. Based on mathematical statistics analysis, this paper also proposes the seasonal characteristic, daily characteristic and probability distribution of wind power and photovoltaic output that help simulate the operation of wind-solar-hydro-thermal joint operation power grids in different spatial and temporal scales and determine the reasonable configuration of all kinds of power supply and economic operation scheme. The analysis shows that the relevance between the output of photovoltaic power and the time is strong while the relevance between the output of wind power and the time is weak. And the complementarity between hydropower and photovoltaic power is obvious while the complementarity between wind power and photovoltaic power isn’t. For wind-solar-hydro-thermal joint operation power grids, it’s inappropriate to use simple peak load balance under light load flow mode to estimate its acceptance ability of new energy. We should determine the reasonable installed capacity and structure of all kinds of power supply through the operation simulation of the power system.