【Background】The radiation of incident solar rays is the key factor affecting the plant canopy spectral reflectance. This study aimed to evaluate the impacts of environmental conditions on the canopy spectral reflectance by the invasive aquatic weed, Alternanthera philoxeroides, thus to establish the appropriate conditions for multispectral radiometer usage.【Method】The canopy spectral reflectance of A.philoxeroides under different weather condition, instrument lens heights, solar elevation angles and leaf moisture levels were measured by a multispectral radiometer.【Result】All of the four factors examined affected the canopy spectral reflectance significantly at 560 nm within the green light region, 660 nm within the red light region and 810 nm within the nearinfrared light region.【Conclusion and significance】The results indicated that the spectral scanning experiment should be carried out during 10:00 to 14:00 am in sunny days and the height of radiometer lens to canopy should be kept at 1.3 m above canopy, thus to ensure modest solar elevation angle and dry plant leaves, because the plant canopy spectral reflectance in these conditions can keep steady. The research could provide a basis for improving evaluation technology of the canopy spectrum.