The Bohai BZ gas field has the characteristics of deep burial, high temperature and pressure, and diverse fluid types. Below the dew point pressure, the phase state and composition of the fluid undergo extremely complex changes. The characterization of this complex fluid phase is very difficult, which seriously hinders the formulation of development and production measures. We have developed a systematic solution for fluid phase analysis, which is divided into three parts. 1 Regarding the reliability of PVT experimental data, analyze the principles of constant mass expansion and constant volume depletion experiments. Summarize the methods for determining the qualification of PVT experimental data based on principles. Combining state equation analysis, form a technical process for characterizing the phase state of single-phase fluids. 2 Conduct theoretical research on phase recovery for the characterization of oil and gas composite fluids. Fit fluid characteristic parameters through PVT experiments to obtain the phase characteristics of composite fluids. Finally, accurately characterize the distribution of oil and gas. 3 Conduct experimental research on pressure reduction and depletion in response to non-equilibrium phase changes in fluids. Describe the process of non-equilibrium phase state changes by correcting the equilibrium constant. Characterizing the mechanism of foggy flow seepage through equivalent interfacial tension coefficient. The technical system has been applied as an example in the Bohai BZ gas field, and the following meaningful conclusions have been obtained. 1 By using the PVT experimental data discrimination method, three unqualified samples were excluded, providing effective assurance for phase characterization. 2 By using the composite fluid phase characterization method, it is confirmed that Block 26 of the BZ gas field is a condensate gas reservoir with an oil ring. By characterizing the distribution of oil and gas, we guide the addition of two adjustment wells, resulting in an increase of 160,000 cubic meters of oil production. 3 Accurately fit the gas oil ratio pattern through non-equilibrium phase characterization. Establish a control method for gas wells in highly saturated condensate gas reservoirs, guide 15 well operations, and increase oil production by 22,000 cubic meters annually. The BZ gas field has proven the effectiveness of this technical process and can serve as a reference for the development of similar complex oil and gas reservoirs in the industry.