Air side heat transfer and fluid flow characteristics of four-row slotted fin surfaces with arc-type and X-type strip arrangement in pressurized air cooler were studied by visualization experiments.Experiments were carried out with PIV（particle image velocimetry）system and infrared thermo tracer to visualize the flow field and the temperature distribution of two slotted fin surfaces.The experiment results showed that the fluctuation intensity and pressure drop of the X-type slotted fin surface were higher than those of the arc-type slotted fin surface.However，the heat transfer performance and field synergy property of the X-type slotted fin surface were superior to those of the arc-type fin surface.A new definition of fluctuation intensity was proposed to indicate the magnitude of flow disturbance enforced by the configuration and arrangement of slotted fin surface.The higher the fluctuation intensity，the larger the velocity gradient and the vorticity magnitude.The visualization experiment results provide a reliable experimental support for later numerical simulation and optimization design for new fin structures.