Physical characteristics of the initial breakdown pulse has been studied widely,but the exact three-di-mensional position of the initial breakdown pulse and physical characteristics of the maximum amplitude initial breakdown pulse are not understood deeply yet. Based on three improved transmission line models and particle swarm optimization algorithm, the bipolar pulse of initial preliminary breakdown process in different types of lightning at different distances is fitted by using high-precision lightning three-dimensional radiation source localization results and synchronization data of wideband electric field pulse waveform at Da-tong of Qinghai. The radiation field component is modified and the current waveform of initial streamer channel and characteristic parameters are derived. The fitting effect of three models on the bipolar pulse waveform of the initial preliminary breakdown process and step leader process are comparatively analyzed, and the physical feature of the initial breakdown pulse are statistically analyzed. At the same time,the bipolar pulse propagation mechanism and physical characteristics of the lightning initial pre-breakdown process are further studied. All three models can reasonably fit the measured bipolar waveforms. and it may be more reasonable that the current in the channel exponentially decays with height. The leader process and the initial preliminary breakdown process of the negative cloud-to-ground flash show cascaded developing, and both parameters using MTLE model fitting is close, indicating their transmission features are similar. The total a-mount of charge, total vertical dipole moment and the path length of the initial streamer upward propagating to positive charge region are larger than that of the initial streamer downward propagating to positive charge region. It may be related with the distance between the positive and negative charge region. When fitting the bipolar pulse waveform with three improved transmission line models, no model is constantly better than the other models, and it depends on specific analysis of the pulse structure. It should be noted that some of the initial pre-puncturing process bipolar pulse sometimes superimposed on the high frequency pulse, which may affect the fitting effect, but these pulses are considered inevitable product of the lightning discharge process. The presence of these pulses cannot be ignored when fitting. The wide-band electric field pulse waveform produced by lightning discharge is also distorted by the influence of environmental factors (mountains, trees, etc.) during the transmission process, which may cause the fitting effect of the pulse rising edge and the pulse end period not ideal, and therefore,synchronized electric field pulse waveform data in other observation stations can be selected for comparison and confirmation.