Ambient irradiance is pivotal to be considered for field measurements of soil emissivity with Portable Fourier Transform Infrared Spectro-radiometer (102F). Usually, a diffusely reflecting gold plate which has a near-Lambertian behavior was used to acquire the ambient irradiance. Because of the measurements of soil and ambient irradiance are not synchronized, It can generate errors on determination of soil emissivity, especially for the erratic cloud and instantaneous wind which can make the ambient irradiance a sharp change. In this study, four conditions about the ambient radiances were 30% underestimated, 50% underestimated, 30% overestimated and 50% overestimated to assess the impacts of ambient irradiance on determination of soil emissivity. Preliminary research shows that ambient irradiance has more impacts on determination of soil emissivity in 8-10um than it in 10-13um. In 8-10um, the relative difference of soil emissivity can be more than 0.005 when the ambient irradiance was 30% overestimated. And it can reach up to 0.01 when the ambient irradiance was 50% overestimated. The error magnitudes are related to soil types. By contrast, the impacts of ambient irradiance are not obviously in 10-13um. Similar results can be seen in the ambient irradiance were underestimated conditions.