The leader-streamer propagation is one of most important stage in long air gap discharge. In the conductor-tower lattice configuration, we have measured the voltage, current on high voltage side and electric field in the gap. While the streamer in the leader-streamer system presented a conical or hyperboloid diffuse shape, the clear branch structure streamer in the front of leader was firstly observed by a high speed camera in the experiment. Besides, it is found that the leader velocity, width and injected charge for branch type streamer are greater than those of diffuse type. We propose that this phenomenon results from high humidity which was 15.5~16.5 g/m3 in our experiment.