Deep and ultra-deep petroleum resources have become major contributors to petroleum reserves. The Tarim Basin has recently witnessed discoveries of several oil reservoirs at depths exceeding 8000 m, which extend the exploration depth limit for crude oil and light oil resources. To clarify the role of overpressure during the critical stage of crude oil cracking (Easy Ro ≈ 1.0–2.0%), this study conducted low-temperature, long-duration, overpressure (150 MPa) gold tube pyrolysis experiments on marine crude oil from the Tarim Basin. Comprehensive analysis of the cracking products (C₁–C₃₀₊) revealed significant differences in the thermal stability and cracking behavior of hydrocarbon molecules with different chain lengths: long-chain hydrocarbons (C₁₂₊) were continuously consumed as the primary reactants, whereas short-chain hydrocarbons (C₆–C₁₂) initially formed as products and subsequently underwent secondary cracking as reactants. During this process, overpressure played a critical role in delaying the yield peak of light hydrocarbons and suppressing their secondary cracking. This mechanism resulted in a slower increase in gaseous hydrocarbon yield under overpressure conditions, and the carbon isotopic composition clearly recorded a shift in the cracking precursors from heavy to light hydrocarbons. Furthermore, fluorescence lifetime, as a sensitive spectroscopic indicator, exhibited delayed decay under overpressure, confirming the inhibition of aromatization and polymerization reactions by overpressure. By illuminating the sequential nature of hydrocarbon cracking and the moderating influence of overpressure at molecular and spectroscopic levels, this work offers crucial evidence for understanding multi-phase hydrocarbon coexistence and forecasting the preservation depth of discrete-phase crude oil in the Shuntuoguole area.