Heat conduction, water flow, solute transport, and gas diffusion occur concurrently in unsaturated soils. Yet there lack reliable methodologies for quantifying these highly coupled processes. Previous studies have confirmed that the thermo-time domain reflectometry (TDR) can be applied to monitor soil water content (θ), thermal conductivity (λ), and electrical conductivity (σ) under field conditions. In this study, we further extended the thermo-TDR technique for determining soil water retention curve (SWRC) and gas diffusion coefficient (Ds) of sandy soils. SWRCs were obtained from measured λ(θ) curves using the Liu et al. (2024) model, along with the information of soil texture and bulk density. A piecewise model was developed to estimate Ds from SWRC data across three distinct regions that were divided by the air entry value and the wilting point (-1500 kPa). The thermo-TDR based approach was validated against direct measurements on a sand and a sandy loam with θ ranging from 0.02 m³/m³ to saturation and bulk density ranging from 1.35 to 1.60 Mg/m³. Results showed that the Liu (2024) model provided accurate SWRCs with RMSEs < 0.055 m³/m³ for θ data. The Ds values derived from SWRC data agreed well with the measured values, with a RMSE of 0.0080 cm²/s. Thus, the thermo-TDR technique can be used for quantifying the dynamics of water availability and pore-size distribution, especially in soils with unstable structure.