Abstract: Under the background of global warming, the frequency, intensity and impact range of extreme high temperature weather continue to expand. Coupled with the continuous increase in the number of air conditioners and the increasing proportion of installed capacity of wind and solar power, the impact of extreme high temperature on the power supply-demand of new power systems is becoming increasingly significant. A probabilistic assessment method for supply-demand balance capacity of new power systems is proposed and applied to the medium and long-term power system production simulation. Firstly, a set of joint wind-solar-load scenarios is constructed based on a three-dimensional Gaussian Copula function to characterize the relationship between variables. Then, integrated with a power system production simulation model, the dispatch strategies of generation units and the utilization of demand-side resources are optimized aiming at minimizing the operational costs. Finally, an assessment index system for supply-demand balance capability is established, encompassing the dimensions of security & adequacy, flexibility & controllability, cleanliness & low-carbon, and economic efficiency.The analytic hierarchy process (AHP) is employed to set the weights for these indicators, and the kernel density estimation (KDE) is adopted to generate the probability density curve of the supply-demand balance capability, which reveals its probabilistic distribution characteristics and quantifies the likelihood of occurrence for different levels of balance capability. This method is applied to a province in South China to conduct studies for the years 2030, 2040, and 2050. The results indicate an overall enhancement in the supply-demand balance capability of the new power systems, primarily driven by improvements in the flexibility & controllability and cleanliness & low-carbon dimensions. The specific outcomes of the province by 2050 include an increase in the utilization rate of pumped storage to 34.6% and a reduction in the carbon emission intensity per unit of electricity to 0.27 kg/(kW·h). Conversely, a decline is observed in the security & adequacy and economic efficiency dimensions. By 2050, the average reserve margin is projected to decrease to 7.1%, while the average levelized cost of electricity is expected to rise to 0.56 RMB yuan/(kW·h)