The upper part of the transmission tower has cross-arms with complex geometric shapes and large mass, which is the key part of the wind-resistant design of the transmission tower. However, the wind-induced vibration coefficient in the design wind load codes does not accurately consider the wind-induced response caused by cross-arms. Utilizing the effective static load distribution method, a step-by-step calculation method was developed for determining the DWLs on transmission towers, accounting for the influences of cross-arms. The background equivalent static wind load and the resonant equivalent static wind load of the equivalent static wind load are derived and investigated, respectively. To calculate the background response and resonant response, a correction coefficient was introduced for the wind-induced vibration coefficient to consider the influence of shape on the spatial correlation of the fluctuating wind, and a correction coefficient was introduced for the resonance component factor to consider the spatial correlation of the fluctuating wind. By introducing these two coefficients, the cross-arms wind-induced response can be accurately calculated. When the calculation model is consistent with the actual transmission tower, the design wind load formula involves a large number of parameters and complex multiple integral expressions, so the intermediate variable is introduced to simplify it. The design wind load formulae of the transmission towers with cantilever cross-arms were derived by analyzing the design of 6 tower samples. Furthermore, the time domain calculation of an actual transmission tower case verifies the design wind load formulae derived in this paper. The parameter analysis of the design wind load shows that the cross-arms of the transmission tower accounts for the main part of the contribution of the resonant equivalent static wind load to the wind-induced vibration coefficient. In the transmission tower with cantilever cross-arms, the resonant equivalent static wind load greatly influences the wind-induced vibration coefficient, but the background equivalent static wind load has little effect on the wind-induced vibration coefficient.