In-site experiments are conducted to simulate a group of wheat colonies with gradually decreasing Leaf Area Index (LAI) by removing individual wheat plant from an original wheat field. The LAIs vary from the original 5.2 (sufficient saturation) to 0 (uncovered soil) at the step of 0.52 or 10% of the original one. The potential and reliability of using remote sensing method for crop growing assessment was discussed. Taking the curve of uncovered soil's reflectance spectra as a reference line, and using the distance between the curve of the reflectance spectra at a specific LAI level and the reference line as a measure, at the locations of three feature spectra in visible and infrared band, i.e., 680nm, 890nm and 1460nm, the response of reflectance spectra of wheat colony to the variance of its LAI was studied. The results show that with the reduction of LAI, the maximal closing motion of wheat colony curve to the soil reference line occurred at 890nm, followed by 1460nm and 680nm. And when LAIs decrease gradually from 5.2 (100 percent of wheat colony) to 4.68(90 percent colony), to 4.16 (80 percent colony), and finally to 0, the approaching speeds of wheat colony spectral curves to soil spectral curve are different. In the range of 5.2(100%) to 2.6 (50%), the approaching speed is smoothly low, especially in the range of 4.68 (90%) to 2.6(50%), while the closing speed in the range of 2.08 (40%) to 0.52 (10%) mount to very high.