Study on feed intake of phytophagic animals is a key issue in promoting animal productivity and conservation of wild life. However, how to accurately predict the feed intake of grazing animal and wild life is a long remaining problem. Under the mechanism of co-evolution, plant produces secondary compounds such as phenolics, terpenoids and nitrogen-containing compounds to avoid or reduce animal herbivorous damage as a defensive strategy, while animal attained detoxification capacity of biotransforming and mineralizing the compounds by microbial activities and reactions such as hydrolysis and reduction. The attributes of feedstuff and the amount of a particular feed consumed by the animal affect directly the urinary excretion of secondary metabolites. Plant secondary compounds and their metabolites can be efficiently extracted, separated and structure-identified by spectroscopic analytic method. Then the feed intake of the animal can be accurately measured or predicted by the inference model of concentration-ratio that is based on the regression of correlating the secondary metabolites to the precursors in plant. Aromatic compounds, an universal occurrence in vascular plants, play an important role in predicting feed intake of ruminants. Progresses have been made all-around about the new method. Intensive studies have found that different species and developing stage of plant have varying kinds and levels of secondary compounds, and the age, gender and type of animal have different capacity of metabolizing the compounds. Increasing concentrations of the compounds in the diet led to a dose-dependent decrease in food intake best described as an exponential decay. Animals that had not previously been exposed to the compounds ate significantly more when first offered food containing the compound than on subsequent days. Advanced spectroscopic analytic method has been developed and widely applied in extraction (e. g. microwave assisted extraction and ultrasonic extraction), separation and purification (e. g. paper chromatography, VLC, GC, HSCCC, Micro-LC and HPLC), and structure-identification (e. g. Fourier transform infrared spectroscopy, ultraviolet spectroscopy, and nuclear magnetic resonance spectroscopy) of plant secondary compounds and their metabolites. Studies suggest that some aromatic compounds like phenolic alkaloids, flavonoids, tannins, lignin and N-alkane are suited internal markers and find that the method to predict animal feed intake of plant secondary compound by spectral analysis is quick, accurate and applicable. The further focus should be on selecting appropriate compounds and their fate in metabolizing and excretion, and the development of intelligentized spectroscopy equipments.