In this study, the extraction characteristics of 1-phenyl-3-methyl-4-benzoyl-pyrazolone-5 (abbreviation: HPMBP) are studied. HPMBP can exist in four tautomeric forms. Gaussian computations demonstrate that the order of thermodynamic stability of the isomers for HPMBP in xylene is keto(II) > keto(I), enol(III) > enol(IV), and enol(M) > keto(II). HPMBP in xylene mainly exists as the keto(II) and enol(III) isomers. When the concentration of HPMBP is beyond 0.1 M, HPMBP in xylene mainly exists as the enol(III) isomer. In xylene, the tautomeric equilibrium constant between the keto(II) and enol(III) isomers is 9.42. Our experiments show that copper ions could be extracted not only by the enol(III) isomer but also by the keto(II) isomer of HPMBP. When the concentration of HPMBP is relatively high, the enol(III) isomer extracts a high percentage of Cu(2+), whereas when the concentration of HPMBP is very low, the keto(H) isomer extracts a high percentage of Cu(2+). When the keto(II) isomer is used as extractant, the extraction of copper ions complies with an ion-pair mechanism. When the enol(Ill) isomer is used as extractant, the extraction of copper ions complies with an ion exchange mechanism.