In this work, we propose a full current model in order to interpret the mechanism of terahertz (THz) wave generation from single-color femtosecond laser filamentation under transverse and longitudinal direct current fields. The model consists of two processes, i.e., oscillation of the microscopic plasma current and radiation of the macro current transmission line. Both processes are combined to describe the terahertz (THz) radiation characteristics such as terahertz wave enhancement and spatial distribution evolution. Compared with the well-known transition-Cherenkov radiation theory, the proposed full current model can achieve phase matching under the condition of equal light speed, and has a clearer physical picture and simpler formulas. Moreover, it can reproduce the experimental results well.