In order to understand the effect of city expansion on the soil organic matter (SOM) changes in suburban areas, GIS technology and geostatistics method were used to study the spatial and temporal variation of SOM in Shanghai and the driving factors were discussed. The results showed that SOM in the study area had a moderate variation. SOM content in the southwest areas was high but low in the eastern areas. SOM content increased from 1980 to 1999 but decreased from 1999 to 2010. From 1999 to 2010, when paddy fields converted to the vegetable fields or nursery fields, SOM content decreased 6.72 g/kg and 6.12 g/kg, respectively, but when the paddy fields kept unchanged, SOM content only decreased 1.51 g/kg. Spatial overlay analysis showed that SOM content reduced in most of the area, only increased in some areas in the north, south and southwest. The areas where SOM increased were far away from the city and the areas where SOM decreased were near the city. The main reason was when some paddy fields which near the city converted into vegetable fields or nursery fields, crop residues and straw returned decreased and the decomposition rate of SOM increased, which led to a decline of SOM content.