The diffusion of fuel spray in swirl flow is vital for the combustion of diesel engine, however, the researches on this is still mysterious due to the obstacles on direct investigations on a real engine. The research of intake swirl in engine at present normally use CFD simulation or based on data analysis of combustion and exhaust emission, the specific mixing process of fuel in swirl flow still not very clear. In this paper, a rapid compression machine (RCM) with an optical combustion chamber was established with the mean compression velocity of 7.55 m/s. Three kinds of flow fields, including intense swirl field, weak swirl field and turbulent field, were conducted to investigate the diffusion characteristics of injected fuel, the mixing processes of fuel spray in the swirl flow fields were recorded by the high-speed camera. Experimental results show that the mixing of fuel spray in air consists of four stages: fuel injection, wall approaching, rapid diffusion and final diffusion. The fuel spray is firstly bended by the lateral flow of the swirl field, then liquid fuel is blown onto, and coats on, the sidewall. The fuel on the sidewall is finally enrolled into the chamber plenum afterwards in gaseous phase via the spiral arms (in swirl fields) or random routes (in turbulent field) At last the fuel disperses all over the chamber. The results give an intuitive observation and help better understand the air-fuel mixing process in diesel and direct inject gasoline engines.