This paper studied the effect of different periods of plastic film mulching (M0-no mulching, M30-mulching for 30 days, M60-mulching for 60 days, and M-mulching for whole growth period) on soil microbial biomass carbon (SMBC) of spring wheat field in semi-arid loess plateau. The mean SMBC in 1999 and 2000 was 335.3 and 259.3 mg.kg-1 dry soil, respectively. It was 29.3% higher in 1999 than in 2000. The highest SMBC was recorded at the harvest stage in M treatment for the two years. In 1999, a wet year with more rainfall, the SMBC of M60 and M treatments was significantly higher than those of M0 and M30 in the mid-period of growth, reached its top at the end of the growth period. The highest grain yield was also achieved in M60. It was a dry year in 2000, but rainfall was rich in the latter growth period of spring wheat. SMBC increased at the beginning period of growth, and did not increase during the mid-growth period. It increased again during the latter period of growth, and showed a significant difference among the treatments. At the harvest stage of 2000, SMBC in M0 was the highest among all the treatments. It was similar between M and M60, and lower than that of M30. In the two years, the ratio of C/N ranged between 7.732-9.042, being lower than the threshold of 11.3, and the ratio of C/P was 300.8-719.6, being higher than the threshold of 300. The two parameters showed that the increase of SMBC was inhibited because of the competition of soil available nutrients between soil microbes and crops. These indicated that soil organic matter content was so shortage that it became the key factor to restrict crop productivity. Under this condition, increasing crop productivity through the input of chemical fertilizers would conceal the problem of soil degradation, and result in a further decrease of soil quality. A long term plastic film mulching would make the problem more serious.