Herein, global sensitivity analysis is performed to analyze the numerical model of the thermal blooming distortion parameter (ND) with 113 uncertain input factors in typical transmission scenarios. First, the elementary effect test ranks factors in order of their importance and screens insensitive factors. Next, variance-based sensitivity analysis quantifies the sensitivity of the important factors and investigates the interactions between factors. Our results reveal that the atmospheric characteristics in the propagation path’s first few kilometers affect ND the most. These characteristics can either affect ND alone as well as with other factors. The most important parameters are atmospheric absorption coefficients at low attitude. These findings can guide the measurement and model construction of atmospheric parameters.