Abstract: During the process of cloud-to-ground lightning connection, the propagation of downward leader to the near-ground area can elevate the electric field at one or several points on the surface of ground tip object to the breakdown threshold of surrounding air, initiating one or more upward leaders, which are known as multiple upward leaders. The emergence of tall buildings has led to an increase in the number of observations of upward lightning strikes on different buildings or the same building. The presence of multiple upward leaders means that multiple parts of the building may be struck. Conducting simulation experiments to study the mechanism of the multiple upward leader phenomenon is of great significance for developing lightning protection. The relative velocity ratio of the downward and upward leaders may be one of the key factors in the lightning connection process. The relative speed ratio of leader propagation in random lightning connection mode cannot accurately describe the relative distance ratio of downward and upward leader propagation. Taking into account the optical observation facts and the electric field environment during thunderstorms, the background electric field module setting is improved on the basis of the existing three-dimensional random mode for multiple upward leaders. It also incorporates a relative propagation speed module for the downward negative and upward positive leaders, establishing the relative propagation speed of leaders according to their propagation distance. Applying the new model to simulate multiple upward leader phenomena triggered by a flat-roofed single building, compared with the previous version, parameters of the new model, such as flash distance and upward leader length, show better consistency with natural lightning. On this basis, the lightning connection process on the high-rise buildings in the Pearl River New Town is simulated, and the improved model can more accurately replicate the lightning occurrence patterns of complex buildings. Characteristic parameters of lightning strikes on urban building clusters are mainly determined by factors such as the shape characteristics, relative position, and relative height of each building. The distance at which lightning strikes buildings is positively correlated with their height. The probability of lightning strikes, the distance of lightning strikes, and other parameters of buildings with similar shapes in the same building group are relatively consistent during ground lightning activities. However, there are still special events that occur when a branch of the downward leader is in close spatial proximity to the building, causing the upward leader to initiate at the top of the building and connect to it.