In order to augment the precipitation on the ground, cloud seeding experiments have been carried out for more than 60 years. However， it remains to be very difficult to evaluate the seeding effect because of the large nature variations of the cloud. By the use of GPS and CINRAD techniques, a physical test was made for the operation of weather modification in Guangxi on 16 February, 2008. It shows that after the cloud seeding, microphysical changes in the supercooled clouds may lead to the enhancement of the basic reflectivity on radar in both of the supercooled cloud layer and the warm cloud and rain layer, and be more quickly in reaching the maximum value than the nonoperated clouds which are under the similar conditions, and last for a longer period. After the cloud seeding, the maximum enhancement rates of the echo reflectivity in the supercooled cloud layer and in the low and warm region may reach 200% and 100%, respectively.