To explore the initiation and propagation of upward leader (UL) in the cloud-to-ground lightning attachment process, on the basis of the existing multiple upward leaders (MULs) three-dimensional model and considering optical observations and environmental electric field, the background electric field module is implanted and the UL and downward leader (DL) modules are improved. Sensitivity experiments are then carried out using the improved model. The results are as follows: (1) The new model can simulate a variety of DL, and the number of its branches is greatly reduced. Also, the model can well reproduce various cloud-to-ground lightning attachment processes. (2) Statistical analysis of simulation results shows the UL length, striking distance and the frequency of MULs originating from a structure are positively correlated with the structure height. (3) In addition, as the height of the building increases, the initial time difference between the MULs that start successively in the same lightning attachment process shows an overall upward trend. Preliminary research results indicate that the structure height has a significant impact on the initiation of MULs. And due to the influence of the structure height and the positive and negative leaders, MULs triggered by a low structure start almost at the same time, while on the tall structure, the first UL can propagate a certain distance before initiating subsequent UL.