The two-phase flow in the primary converging-diverging nozzle of a transcritical CO2 ejector was investigated by a visualization measurement experiment using the direct photography method. The transition of the phase change position in the primary nozzle was discovered experimentally and analyzed theoretically based on the isentropic expansion process. The pressure distribution along the primary nozzle was measured by the pressure sensors simultaneously under different operating conditions. The expansion state of the primary flow and the nozzle outlet quality were studied by comparing the nozzle outlet pressure with the suction flow inlet pressure. The visualization images revealed that the phase change could start after or before the throat according to the operating conditions, which was consistent with the isentropic analysis results. In addition, the phase change position moved upwards when the primary flow inlet pressure and temperature decreased simultaneously. The pressure measurement results found that the nozzle outlet pressure was obviously higher than the suction flow inlet pressure in the experiment, which indicated that the primary flow was under-expanded in the primary nozzle. The nozzle outlet quality calculated with the nozzle outlet pressure was less than that calculated with the suction flow inlet pressure. The visualization experiment can be used to improve the CO2 ejector geometry and provide useful data for CO2 ejector model validation to better interpret the internal complex flow phenomena.