The invention provides an on-line analyzing method of a terephthalic acid crystal particle diameter in a P-xylene oxidation process. In a P-xylene (PX) oxidation operation of an Invista technology for production of terephthalic acid (TA), on-line control is carried out on a TA crystal particle diameter; a process operation parameter of a TA series crystallizer is selected, wherein the process operation parameter affects the TA crystal particle diameter, and normalization is carried out on the parameter; a BP neural network model is utilized to carry out simulation calculation; after reverse normalization is carried out on a model output value, a manual analysis value of a TA crystal median particle diameter is utilized to carry out on-line correction on model output that has been processed by reverse normalization, so that a soft measurement value of the TA crystal median particle diameter is obtained; and according to the soft measurement value of the TA crystal median particle diameter, real-time deduction and control is carried out on the TA series crystallizer; that is, a TA crystal particle diameter that is obtained by filtering processing according to the soft measurement value is used to adjust a crystallizer operation parameter, so that deduction and control on the TA crystal particle diameter is realized.