Hainan is located in the southernmost part of China,at 18 ^· 10' -20 ^· 10'N and 108 ^· 37'-111 ^· 03' E.The coastline of Hainan Island is 1,528km.The area of the island is 33,920km² ,accounting for 42.4% of the total tropical area of China(80,000km² ).Hainan is the second largest island in China ,next to Taiwan.

Hainan Island used to historically boast dense original tropical forests due to its special geographical environment. Before 1938, the covering area of the tropical forests in the island made up 46% of the total area of the island. In the period of past half century and over, the forest resources here were destroyed severely because of felling a great deal of trees by humankind. At present, the covering area of the forests in the island accounts for only 1.89% of the total area of the island. Therefore, it is an important measure to protection and development of the tropical forests to provide, accurately and in time, the monitoring information on local forests for the governmental departments concerned by using an advanced technique.

In the present study, the data source from LANDSAT-5TM was used as a major information source. The earth-surface objects in the forest regions were classified by using the combined characteristics of the wave spectra from the earth-surface objects as a classification evidence and in accordance with the possibility of the present remote sensing technique and systematic software as well as the requirements of the services and tasks concerned. 15 types are defined .They are virgin forest ,many-year-long secondary forest ,median-class secondary forest, low-quality secondary forest ,bush ,artificial forest ,grassland, agricultural land, heavily burnt forest ,lightly burnt forest ,heavily bare land,lightly bare land , quarry ,water body and settlement.

The earth-surface objects are usually characterized by their own distinct reflection and radiation in wave spectra. The radiation and absorption of an electric magnetic wave from a substance are caused by vibration of atoms ------ the smallest micro-particles constituting a substance, molecular rotation and transition in energy level of electrons. In conversion of a high energy status into a low one, energy should be released and electric magnetic wave be radiated of a certain wavelength; in conversion of a low energy status into a high one, energy should be absorbed from the outside. Therefore, any objects themselves have the capability and characteristic of radiating, absorbing and reflecting an electric magnetic wave. So, different objects may be distinguished according to the difference in the characteristics of the spectra of electric magnetic waves received by a remote sensing instrument. The determination of the brightness values of the wave spectra from the various forest regions within the limits of the wave bands 1----7 ( except the wave band 6 ) in the stretched images obtained each year was carried out by using a computer. The reflecting rates of the brightness were calculated by the following equation:
R -reflecting rates of the brightness
B -image element brightness value

It is discovered through comparison that the variation of the reflecting curve of lightness value(100%)of different earth-surface objects is very visible(shown in the figure)