Floating conductor has an effect on the process of air gap discharge, a laboratory experiment has been designed to analyse in detail the development of such discharge in a rod–plane air gap, submitted to an impulse voltage. High-speed optical observation technology, discharge current measurement technology, and the method for the synchronisation of the above technologies were used in experiments. Experiments obtained the key physical parameters accurately and the authors give a detailed description of the observed phenomena. The coupling mechanism between the positive and the negative discharges have been studied according to the temporal evolution of the floating potential of the conductor. Based on it, a physical model including the process of the first corona initiation and the streamer development, leader inception, the streamer-leader system propagation, and final jump is proposed. These results will be used for the development and validation of the model of discharge process to obtain numerical simulations of the rod–plane air gap discharge with a floating conductor.