To understand the behavior of molybdenum and tungsten extracted by tri-alkyl phosphine oxide (TRPO) from peroxide solution, the extraction mechanism was studied by slope method and Raman and FTIR spectroscopy. The empirical formulas of molybdenum and tungsten extraction distribution ratio (D_Mo and D_W) as functions of equilibrium pH, TRPO concentration and temperature were obtained by establishing mathematical models. Furthermore, the reliability of the empirical formula was verified in the H⁺-W-Mo-H₂O₂ solution. The results indicate that the calculated values of D_Mo or D_W were consistent with the experimental values. The apparent extraction equilibrium constants of molybdenum and tungsten were =8.51×10³ (0.74≤pH_e≤1.7), =99.89×10³ (1.7<pH_e≤4.62) and =2.65×10³ (0.92<pH_e<2.16) at 20 °C, respectively. The main extraction complex of molybdenum or tungsten was [H₂(Mo or W)₂O₃(O₂)₄(H₂O)₂]·2TRPO. These empirical formulas can be used to analyze and estimate the extraction and separation of Mo and W from low molybdenum and tungsten concentration solutions.