The invention discloses a method for preparing a monoclonal antibody for a nonyl phenol isomeride mixture. NP structural analogue and carrier protein conjugate is taken as the monoclonal antibody for a complete antigen immune mouse to obtain the NP resisting isomeride mixture. The monoclonal antibody obtained with the method is high in specificity and sensitivity, wide in application range and especially suitable for NP content detection and has broad application prospects.