The results of degradation of butyl benzyl phthalate(BBP) by activated sludge demonstrate that the activated sludge is a high efficiency flora for degrading BBP, which can degrade BBP quickly and more than 95 % BBP is degraded in one day. The degradation of BBP can be described by the first-order dynamics model, the concentration in range of 100,300,500 mg x L(-1), the dynamics equation of degradation respectively is Inc = - 0.0699x + 5.6171, nc = 0.0683x + 8.4083, and Inc = - 0.0583x + 7.2806, t1/2 is 9.92, 10.15, 11.89 h, the constant of degrading rate kb is 0.0699, 0.0683 and 0.0583 d(-1). The constant of degrading rate decreases as the concentration of BBP increases, which shows that BBP of high concentration inhibits its biodegradation. In addition, two degraded productions, monobutyl phthalate and methanoyal phthalate, are identified from the degradation of BBP with HPLC-MSn. The route of degradation is that BBP produces mono-ester firstly, and then the product turns into phthalic acid, finally it oxidizes into CO2 and H2O.