The combustion process of internal combustion engine is greatly influenced by the swirl flow in combustion chamber, especially for diesel engines, where the diesel diffusion process in the swirl flow field is vital for combustion efficiency and the formation of pollutants, including nitric oxides and particulate matter. However, the spray diffusion process has not been lucubrated due to the difficulties of direct observation and the characterization of the internal flows in engine chamber. The diffusion characteristics of diesel spray in different intensity swirl flows were studied visualized. In this experiment, the combustion chamber of the internal combustion engine was simulated by a visualized rapid compression machine, and the diffusion characteristics of the diesel spray under three kinds of flow fields, such as tangential swirl flow field, oblique swirl flow field and straight swirl flow field, were studied. An electronically controlled injector with nozzle hole diameter of 02mm was used. The injection pulse width was 5ms, and the two injection pressures were selected to be 40MPa and 60MPa, respectively. The experimental results showed that the peak and valley values of diesel spray area and window area ratio were the highest in the tangential swirl flow field and the lowest in the straight swirl flow field under the same injection pressure and fuel injection pulse width. The process from the start to the complete mixing of the spray was the shortest in the oblique swirl flow field and the longest in the straight swirl flow field. The diffusion time of the spray oil was inversely proportional to the tangential velocity of the swirl flow and the fuel injection pressure. The results can provide reference for the design of air movement organization in diesel combustion chamber.