Abstract: This paper deals with the decomposition patterns of organic .naterials after addition to soils covered with plastic film during 1985 - 1993 at liaoning province. The characteristics of soil organic matter in covering soils were also studied compared with those of uncovered soils. The results obtained from the long-term experiment with carborundum tubes showed that remaining orgainc matter (Yt) of different organic materials after addition to soils and time (tone year) fitted into the equation Yt=Yoe- t, where Yo means percentage of resistant organic C in total C applied, X is decompositiom rate of resistant organic matter. Nearly all correspoding Yo and Yt decreased in soils covered with plastic film. In addition, the results from the field experiment showed that the relative contents (%) of active organic C in total orgamic C declined, whereas PQ values(the percentage of humic acids in extractive humus) and A2 /A3 ratio (absorhance ratio of humic acid at 200nm and 300nm) as well as the absorbance of humic acids in the wave length of 726~400nm increased respectively.