Taking the guyed portal tower of a 220 kV transmission line as the background, comparative studies on the response characteristics of the guyed portal tower under a downburst and the conventional terrain B wind field were carried out by the time history response analysis. 3-D finite element model of guyed portal tower-transmission line system was established, and the dynamic characteristics analysis was performed. The static force coefficients of the rigid guyed portal tower model and double-split conductor model were obtained by wind tunnel tests. Based on the numerically simulated fluctuating wind load induced by the downburst field and Chinese code defined terrain B wind field, the responses of the guyed portal tower under the fluctuating wind loads were analyzed through the Newmark-β method, which was compared with the results based on the codes defined equivalent static loads. The results show that the peak compressive stress of the tower elements and the maximum tensile stress of the guy wires induced by the downburst are 600 MPa and 1 300 MPa, which are 4.6 times and 2.5 times to the corresponding values induced by conventional terrain B wind field, respectively. In addition, low frequency response components induced by transmission lines play an important role in the whole displacement response of the tower. The wind loads on the transmission lines have great effect on displacement responses, but show little effect on the acceleration responses. Furthermore, comparative study indicates that Chinese code lack the items of the downburst induced wind loads, but the defined loads are appropriate to design the transmission tower under the conventional terrain B wind field, while the downburst loads defined by ASCE guideline may underestimate the destructive effect on the guyed portal tower.