Infrared thermography in outdoor field applications is subject to both the source size effect (SSE) and atmospheric transmission effects, often resulting in an underestimation of target brightness temperature. To address this issue, this study proposes a joint compensation method for infrared radiometric temperature tailored to outdoor observation conditions. This approach first establishes an image convolution filter based on the spatial response characteristics of the infrared system, enabling compensation for radiation diffusion and energy loss caused by SSE. Then, it corrects for atmospheric attenuation and path radiation superposition in the measurement signal through modeling of atmospheric transmittance and path radiance. To evaluate the effectiveness of the proposed method, indoor experiments were conducted using blackbody sources of varying sizes and temperatures to support model training and accuracy assessment. Subsequently, UAV-based infrared thermographic experiments were carried out at multiple observation distances under outdoor conditions to validate the method"s applicability in practical scenarios. Results show that the proposed approach effectively corrects brightness temperature deviations caused by the combined influence of SSE and atmospheric effects. Further analysis under varying observation distances reveals that when the target occupies at least a 4×4 pixel area in the image, the error in compensated radiative temperature can be constrained within ±2%. This work provides a new methodological reference for improving the accuracy of infrared temperature measurements of targets in outdoor environments.