Provided is an isolated CTLA4 monoclonal antibody. The antibody can highly-specifically bind to CTLA4, and can effectively block the binding of CLTA4 to B7, reduce the activity or expression level of CTLA4, relieve the inhibition of CTLA4 on the immune function of an organism, activate T lymphocytes, and effectively treat tumours and immune system diseases. Also provided are a monoclonal antibody conjugate comprising the pharmaceutical composition and diagnostic composition of the antibody, and the use thereof in the preparation of a drug for preventing and/or treating and/or treating in combination tumours and immune system diseases.
