Abstract: Due to the different research angles, researches on the radiation often have different units of the energy, the illumination, and the quantum amounts, which are necessary to be conversed into each other practically. The authors studied the methods of the units in solar radiation measurement, and obtained some conversion coefficients. The conversion coefficients were tested by the actual measured results of solar radiation above and under the different plant canopies using the most advanced radiation instruments made by Li-cor of U.S.A. The results show that the relative errors of conversion above the plant canopies are 7% and 2% in clear and cloudy weather, respectively, which are within the relative error range of the radiation instruments. Under the plant canopies, when the LI-191 and LI-200 are used to sample simultaneously and the results measured by LI-200 are conversed into quantum unit, the average difference between the results of the two sensors during the whole day is 25%, which is much greater than those above the canopies. The great errors of LI-200 sensor under the plant canopies are caused by the facula. The line sensor eliminated the facula effect except when too many are there during the noon period.