Wettability analysis in shale reservoirs is crucial for evaluating reserve assessment. Nuclear Magnetic Resonance (NMR) is a widely adopted non–destructive technique for characterizing wettability in porous media. However, the complex fluid behavior in shale oil poses challenges for traditional T1–T2–based wettability analysis. We developed a T1–T2 relaxation theory specifically in mixed–wettability shale oil, and calibrated the simulation parameters using experimental T1–T2 data from parallel core samples. Furthermore, we conducted numerical simulations of T1–T2 responses at varying conditions, including varying proportions of wetting regions, echo spacings (TEs), contact angles, and magnetic field frequencies (f). We investigated how magnetic field frequency affects T1–T2–derived petrophysical parameters in mixed wetting shale oil. The results show the effects of wettability and frequency on NMR measurements are significant. For example, at TE = 0.4 ms, f = 200 MHz, and a water–wet region proportion of 0.78, the measured porosity (0.217%), T2LM (0.354 ms), and T1LM (27.1 ms) were 0.667 times, 1.79 times, and 2.33 times as high as those at a water–wet proportion of 0.52, respectively. This work provides a novel framework for understanding the frequency dependence of NMR petrophysical parameters in mixed–wetting shale oil, offering valuable insights for both laboratory measurements and well logging applications.