The invention discloses a flexible pressurizing testing method for narrow, small and dense runners. The method comprises the steps of designing a flexible pressurizing device according to a runner dimension, wherein the flexible pressurizing device comprises a plurality of flexible pipes which can be inserted into the runners, a gathering device for gathering the plurality of flexible pipes, and apressuring interface which is welded on the gathering device; respectively inserting the plurality of flexible pipes into the runners; blocking clearances between the flexible pipes and the inner walls of the runners by sodium hyposulfite; blocking one end, which is not inserted into the flexible pipe, of the runner by the sodium hyposulfite, and realizing sealing of a to-be-tested structure after the sodium hyposulfite is cooled and crystal is formed; performing pressurizing testing; and after testing, cleaning the sodium hyposulfite in the runner. According to the flexible pressurizing testing method, the sodium hyposulfite is utilized as the sealing material, thereby effectively controlling blocking depth in the narrow, small and dense runners and ensuring no influence to testing of runner welding seams. Furthermore, after pressurizing, the sealing material which is blocked in the narrow, small and dense runners can be conveniently cleaned and eliminated.