Ice flashover, lightning flashover and birddamage are the main reasons that cause transmission facility failure. The impact of these environmental factorson the operational risk levels of power systems should be taken into account in power system maintenance schedulingand operation planning. This paper studies the midshort-term risk assessment methodology considering theimpact of the external environment. The relationship model between natural disasters and transmission lines is presented.The conditional outage rate model and the sampling technique are then proposed considering the correlatedoutage of multiple transmission lines when a disaster happens. The framework of the mid-short-term riskassessment model is outlined. A test case of Jiangxi provincialpower grid validates the proposed model. The results show that the model can quantify the impact of disasters on the forced outage rate of transmission componentand their outage correlation, and thus effectively revealing the mid-short-term risk of power systems. The model can facilitate a more strategic decision-making on maintenance scheduling and operation planning of power systems.