Outdoor infrared radiometric temperature measurements are affected by both the source size effect （SSE） and atmospheric radiative transfer effects， leading to underestimated target brightness temperature measurements. To address this issue， a joint compensation method for outdoor infrared radiometric temperature measurements was proposed. The method first established an image convolution filter based on the spatial response characteristics of the infrared system to compensate for spatial energy spreading and signal loss caused by SSE. Atmospheric attenuation and additive path radiance effects were then corrected through atmospheric transmittance and path radiance modeling. Indoor experiments using blackbody sources with different sizes and temperatures were conducted for model training and validation. Subsequently， UAV-based infrared thermographic experiments at multiple observation distances were carried out under outdoor conditions to evaluate the applicability of the proposed method. Results demonstrate that the proposed approach effectively corrects brightness temperature measurement errors caused by the combined influence of SSE and atmospheric effects. Further analysis indicates that when the target occupies at least a 4 × 4 pixel area， the compensated radiometric temperature error can be constrained within ±2 %. This work provides a practical approach for improving the accuracy of infrared temperature measurements in outdoor environments.