Belonging to the field of natural polymer degradation, the invention provides a method for ultrasonic assisted photocatalytic selective degradation of condensed tannin and control of the degradation degree. The method includes the steps of: 1) extracting condensed tannin from a raw material containing condensed tannin; 2) preparing the condensed tannin into an aqueous solution, degrading the condensed tannin in the aqueous solution under an ultrasonic assisted photocatalytic condition, and performing sampling at an interval time to obtain a degradation liquid; and 3) controlling the degradation degree of step 2) by formaldehyde binding method, and terminating the reaction of step 2) when the reaction dose of tannin and formaldehyde in the to-be-degraded liquid no longer increases, thus obtaining a condensed tannin oligomer aqueous solution. The method provided by the invention can selectively degrade condensed tannin and also can control the degradation degree at the same time, thus acquiring an ideal polyphenol oligomer with high added value.