<p>The transient temperature field of reinforced concrete members under fire is critical for evaluating their fire resistance. Current methods largely depend on empirical or simplified equations derived from fitting experimental and finite element analysis (FEA) data. However, such equations often lack a sound physical basis and fail to adequately represent the influence of key parameters. Furthermore, existing analytical models typically assume constant thermal properties of concrete and are limited only to single-sided fire exposure, without accounting for double-sided fire conditions, which constrains their practical applicability and predictive reliability. To overcome these shortcomings, this study aims to establish an analytical model grounded in physical principles, capable of accurately predicting the transient temperature field in reinforced concrete walls and slabs under both single-sided and double-sided fire conditions. The proposed approach is based on heat conduction theory and partitioning the wall thickness into a high-temperature region and a low-temperature region. It incorporates the reduction of thermal conductivity in the high-temperature region and accounts for heat transfer between the wall surfaces and the external environment, thereby circumventing the need to directly solve nonlinear heat transfer equations. The accuracy of the model is verified through comparisons with existing experimental data and FEA results. Additionally, a parametric analysis using the model is carried out, which suggests that a reinforced concrete wall should have a minimum thickness of 150&#xa0;mm to avoid failure due to thermal insulation loss in a fire. The analytical model presented here offers theoretical support and practical reference for the thermal response design and fire resistance assessment of reinforced concrete walls and slabs.</p>