Slip velocity of impeller outlet for a double-blade centrifugal pump was studied with PIV technology and slip factors of six blade phases at the seven conditions were obtained. It is found that average slip factor at the six blade phases gradually decreased with the increase of flow rate. Flip factors of the double-blade centrifugal pump were calculated with Stodola formula, Weisner formula and Stechkin formula and all the results were compared with experimental values at the seven conditions. Weisner formula, whose calculation errors were the smallest, was corrected on the basis of experimental values. And a double blade impeller was redesigned to verify correct formula. The results showed that the calculation errors of corrected slip factors at the seven conditions were less than 3.5% and the maximum error was 3.23%.