Continuous power supply of urban power networks (UPNs) is quite essential for the public security of a city because the UPN acts as the basis for other infrastructure networks. In recent years, UPN is threatened by extreme weather events. An accurate modeling of load loss risk under extreme weather is quite essential for the preventive action of UPN. Considering the forecast intensity of a typhoon disaster, this paper proposes analytical modeling of disaster-induced load loss for preventive allocation of mobile power sources (MPSs) in UPNs. First, based on the topological structure and fragility model of overhead lines and substations, we establish an analytical load loss model of multi-voltage-level UPN to quantify the spatial distribution of disaster-induced load loss at the substation level. Second, according to the projected load loss distribution, a preventive allocation method of MPS is proposed, which makes the best use of MPS and dispatches the limited power supply to most vulnerable areas in the UPN. Finally, the proposed method is validated by the case study of a practical UPN in China.