Topsoil samples were collected from unmulched cropland, abandoned cropland and continuous watermelon cropland at different times. The samples were analyzed for soil microbial population and catalase, amylase, polyphenol oxidase and dehydrogenase activities. Then the dynamic characters of soil enzyme activities in crop growth period were studied. The soil microbial population of cropland with gravel-sand mulch was at a low level, the number of fungi decreased significantly, and both of the number of bacteria and actinomycetes in mulched cropland were between those in unmulched cropland and abandoned cropland. Dynamic characters of soil enzyme activities in different treatments were significantly different in crop growth period and however, most of soil enzyme activities reached a maximum value in vining stage or fruiting period. The crop growth and gravel-sand mulch had strong effects on soil enzyme activities. The results can provide a theory basis for the sustainable utilization and scientific management of the gravel-sand mulch cropland.