Abstract: The positive long air gap discharge is the research basis of the external insulation of power transmission and transformation system and lightning discharge physics research, while the streamer to leader transition process has been the focus of the long gap discharge mechanism and the research of lightning upward leader discharge. In order to obtain the evolution law of electrical and thermodynamic parameters and their relationship in the process of streamer to leader transition, the 1.0 m rod-plate air gap was taken as the research object, and the thermal characteristics test observation system for streamer to leader transition process was constructed. The system realized the simultaneous observation of voltage, high potential discharge current, discharge channel optical image, and schlieren image. The high-speed schlieren images were obtained from the beginning of the streamer discharge to the formation of the leader channel. The radial distribution of gas density, gas temperature and their time evolution were obtained by Abel inverse transformation. With the increasement of injected charge of first streamer, the two trends of the discharge channel center temperature from the rise to the decrease in the dark region were obtained. Based on the Gallimberti streamer to leader transition 0-dimensional model, the effects of the initial radius and energy partition coefficient on the temperature variation in the dark region were obtained. It is concluded that the model is only suitable for the discharge in which the temperature rises in the dark period. The research work in this paper can provide reference for the correction of the positive leader inception criterion and the improvement of the physical model.