In order to assess the ecological risks of the toxic red tide alga Alexandrium tamarense to juvenile thick-shell mussels （Mytilus coruscus）， juvenile mussels with an average shell length of （18.40±2.04） mm were selected， and the mortalities of juvenile M. coruscus over 12 days were calculated after continuous exposure to the culture and filtrate of A. tamarense， tissue antioxidant enzymes and metabolism-related enzyme activities were also measured. The mortalities of juvenile mussels were recorded at 48 hours intervals in the control， whole-cell and filtrate groups， and the activities of superoxide dismutase （SOD）， catalase （CAT）， glutamine-phosphatase （GPT）， alkaline phosphatase （AKP）， and acetylcholinesterase （AChE） of the individuals on 4^th， 8^th and 12^th days after exposure were analyzed. The results showed that the corrected cumulative mortality of juvenile mussels in the whole-cell and filtrate groups on the 12^th day was 33.73% and 15.66%， respectively. Enzyme activity trends varied among the groups. The activities of SOD， CAT， AKP and AChE in the whole-cell group showed an overall increasing trend， and the GPT activity showed a decreasing and then increasing phenomenon. In the filtrate group， AKP activity remained relatively stable. After 8 days of continuous exposure， SOD and AChE activities showed a decreasing trend， while GPT and CAT activities increased. By day 12^th， SOD and CAT activities had decreased， whereas GPT and AChE activities had increased. On the 12^th day， the activities of five enzymes in the whole-cell group were significantly higher than those in the control group （P<0.05）. The principal component analysis （PCA） showed that the physiological indicators were positively correlated， and the integrated biomarker response （IBR） index showed that the IBR value of the whole-cell group differed from the control group， indicating that the whole-cell culture of A. tamarense has a high toxic effect on juvenile mussels. The results indicated that both the culture and filtrate of A. tamarense above the baseline density of red tide would stress the survival of juvenile M. coruscus， inducing oxidative stress in mussels， and suppressing the metabolism function of juvenile mussels at the early stage of exposure. This study provides a reference for conducting scientific culture of M. coruscus during toxic red tide outbreaks.