The on-site full-scale test is the key to assess the insulation performance of long air gap. However, due to the practical conditions such as different electrode structures and complex arrangement, typical kinds of gap are always studied as test objects, which is not of university on all practical engineering problems. Scholars at home and abroad are trying to explore the internal relationship between the long and short gap tests. By studying the similarities and diversities of each stages of discharge under different gap distances, a fine grasp of discharge characteristics are achieved. In this paper, a photoelectric joint measurement system composed of photomultiplier tube and electric field sensor is built. The signals of spatial field strength and instantaneous power in the development of 5cm and 3m pin- plate gap discharge under the operating impulse voltage are obtained. The key characteristics of initial streamer start delay, dark period duration, initial streamer electric field jump amplitude and initial streamer optical power jump amplitude are analyzed. The results show that the values of initial time delay, dark period duration and field strength jump amplitude under 5cm and 3m pin-plate gap are similar, and the amplitude of optical power jump is positively related to that of field strength. The similarities are embodied in the initial flow and dark period of discharge process under the long and short gap. In addition, the variance of characteristic parameters of 5cm gap is less than 3m gap’s, and the discharge randomness of short gap is relatively small. In this paper, the characteristic parameters of the discharge development process in 5cm and 3m gap distance are obtained, and the variation rules of the characteristic parameters are analyzed, which provide accurate calculation parameters for the later electromagnetic simulation of long and short gap discharge characteristics.