FreeThaw1D is a fast one-dimensional heat transfer model with phase transition, designed specifically to simulate the heat transfer process in permafrost regions Historical background: This model was developed by the GeoFrame Components Team with the aim of providing an efficient and accurate tool to simulate the heat transfer process in permafrost regions, particularly the impact of water ice phase transition on permafrost temperature and active layer thickness Technical features: Advanced phase change algorithm is used to calculate water ice phase change, considering latent heat exchange during the phase change process. Finite difference method is used to solve the heat conduction equation, with high computational efficiency, supporting multiple boundary condition settings, adapting to different research scenarios, providing flexible parameter configuration, and can be adjusted according to soil characteristics in different regions. Multiple soil thermal physical parameter databases are built in, making it convenient for users to use directly The core function is to accurately simulate the temperature profile of permafrost and changes in the thickness of active layers, monitor and predict the spatiotemporal changes in permafrost temperature, study the dynamic changes in the thickness of active layers and their impact on ecosystems, evaluate the impact of climate change on permafrost, including the degradation process of permafrost under warming scenarios, and simulate the effects of different land use patterns on permafrost thermal conditions Application Case: Simulation of Heat Transfer Processes in Permafrost Regions of the Qinghai Tibet Plateau; Research on the variation of active layer thickness in the Arctic region; Assessment of the impact of engineering construction in permafrost regions on permafrost environment; Prediction of permafrost degradation trend under climate change scenarios; Coupling simulation of carbon cycling process and heat transfer in permafrost regions Limitations: Only considering one-dimensional heat transfer processes cannot simulate lateral heat exchange, and the simulation of soil moisture migration processes is relatively simplified. The influence of biological activities on soil thermal properties is not considered, and the calculation accuracy depends on the accuracy of input parameters Input parameters: thermal physical parameters such as soil thermal conductivity, heat capacity, density, initial temperature profile, surface temperature and temperature fluctuation parameters, soil moisture content and phase transition parameters, simulation time step and duration Output results: temperature time series at different depths, changes in active layer thickness, latent heat exchange during phase transition, frozen soil temperature gradient, heat flux distribution Scope of application: Simulation of heat transfer process in permafrost regions, monitoring of permafrost temperature, study of changes in active layer thickness, assessment of the impact of climate change on permafrost, analysis of the impact of engineering construction on permafrost environment