Traditional statistical methods do not provide accurate prediction for UAV channels under specific scenarios. For the urban scenarios, a geometry-based stochastic channel model for the communication link between UAV-based base stations and ground is developed by a deterministic method based on the ray tracing principle. The proposed model considers the influence of scattering times on received power, and divides arrival rays to the ground into three types, e.g., the line-of-sight component, the single-bounced component, and the double-bounced component. By calculating the electric field along the propagation path and using the ray tracing principle, the computation methods for the path loss and received power of three kinds of components are given. After the digital map is pre-processed, the propagation paths, path loss, power delay profile, and power coverage are simulated and verified. Numerical and analytical results show that the proposed model can accurately reconstruct the propagation situation and the coverage predictions can be used on the UAV layout optimization.