The medium-low maturity shale reservoir is an important unconventional oil resource, and the composition of pyrolysis products can change during pyrolysis development. The changes in fluid compositions in porous media can lead to changes in their phase behavior characteristics, and accurately characterizing pyrolysis fluid phase behavior is of great significance for predicting oil and gas production in reservoirs. In the paper, the Chang 73 medium-low maturity shale were selected and pyrolysis experiments were conducted on shale particles at different temperatures. In addition, an experimental device for studying pyrolysis fluid phase behavior characteristics was constructed, which increased the maximum experimental temperature to 573.15 K compared to traditional devices. Constant mass expansion experiments were conducted on pyrolysis products at five temperatures (293.15–435.15 K) to observe the characteristics of oil-gas interface changes under different temperature and pressure conditions. The P-T (pressure-temperature) phase diagrams were drawn and compared with the calculated results of the P-R (Peng-Robinson) equation. The results showed that as the pyrolysis temperature increased, the lighter component of C3-C12 in pyrolysis oil increased, and the C3-C7 had the highest content at 698.15 K. Under the condition of 698.15 K, the bubble point pressure of pyrolysis fluid products first increases and then decreases with temperature. The P-T phase diagram of experiments has the same trend as the calculated results from P-R equation, but there is also a significant difference. This indicates that correcting the P-R equation is significant, as it is the key to accurately characterize the phase behavior characteristics of pyrolysis products of low-to-medium maturity shale.