Currently difficulties exist still in directly applying urban distribution network risk assessment to the practical early warning and plans. This paper proposes a risk assessment method of urban distribution network under disaster based on minimum over-heated region. By analyzing the outage probability of relevant minimum over-heated region under various disasters, it calculates the outage risk value of loads and applies it as a criterion for timely early warning and plans. This paper takes the self-healing control of distribution network into consideration, overcomes the conservative calculation of traditional risk assessment; significantly enhances the computing speed while quantifies the power loss to compare the extent of damage under different operational states. This paper uses the real urban distribution network as an example to verify the practicality, versatility and high-efficiency of this method.