Soil samples were collected from vegetable fields under greenhouse and upland fields in their vicinity in Yuhong District of Shenyang, Liaoning Province, China, for determination of organic matter, CEC, exchangeable base composition and soil base ion saturation, and hence analysis of effects of vegetable cultivation on soil cation exchange capacity in greenhouse. Results indicate:（1）the protected fields were much higher than the control (upland field) in soil organic matter content, being about 35.6 g kg^-1 and 18.0 g kg^-1 or 2.1 and 1.8 times higher in mean content in the upper and lower soil layers, respectively; and the soil in the former displayed a rising trend in CEC; and an extremely significant and positive relationship between organic matter content and CEC; and (2) in the vegetable fields, total soil exchangeable base increased somewhat in all the soil layers, with exchangeable K⁺, Mg² and Na , doing significanty, and soil exchangeable Ca² remaining almost unchanged; and the former three ions were much higher in saturation degree than those in the control, while the latter was lower in saturation degree and so was the soil base．