The invention discloses a power supply grid model of a urban medium voltage distribution network. A medium voltage public spare line that spare allowance is In is added n the basis of a conventional basic cable grid model; provided that the rated carrying capacity of the maximum single-turn universal trunk line of some city is In, a no-load line that the rated carrying capacity is In can cooperatewith one or more conventional power supply grids in the city for power supply, so that the demand of an N-1 power-losing failure of any user of the power supply grid on the spare allowance can be satisfied, and subsequently, the spare demands of any medium voltage users in the whole city can be met. With the adoption of the medium voltage public spare line that spare allowance is In, the variousconventional basic grid models have the characteristics that the models are universal and the models can be combined and shared. Therefore, the power supply grid model of the urban medium voltage distribution network is universal, expansible, broad in adaption, economical and capable of achieving integration of new and old models and elastic supply; and the grid model has the characteristics of being high in reliability, strong in power supply capacity and low in construction cost.