In order to study the cavitation pattern at impeller inlet of centrifugal pump, a medium specific speed centrifugal pump was selected as an object, and a total transparent water tank was located at inlet pipeline of model pump. Based on pump product testing system and image acquisition system, the cavitation pattern at impeller inlet under three different conditions were measured on the centrifugal pump closed experimental rig. Experimental results showed that the vapors generated at suction side of blade near the leading edge at first, but the location varied with the operation condition. With the decrease of NPSHa, the vapors generated at plurality of blades. The generation, development and collapse process obviously appeared with the rotation of impeller. When the head dropped seriously, the distribution of the vapors hardly varied with the rotation of impeller, and the vapor volume fraction near impeller hub was less than that at shroud side.