Abstract: Gravel-sand mulching is a compreshensive technology in dry land farming that significantly improves microclimatic conditions in arid and semi-arid regions of Northwest China. With the development of watermelon industry and regularization of produce in recent years, continuous watermelon cropping has become a common farming system. The obstacles of continuous cropping of watermelon have been more severe in gravel-sand mulched fields. This study investigated and resolved the obstacles of continuous cropping of watermelon under gravel-sand mulched field conditions. The effects of crop rotation on soil microbial population and physiochemical properties were evaluated based on long-term experiment of continuous cropping under gravel-sand mulched field conditions. The four treatments arrayed in the experiment included watermelon → watermelon (CK), watermelon → kidney bean, watermelon → pepper and watermelon → pumpkin. Results suggested that compared with the control, the three rotation treatments significantly decreased the number and proportion of fungi, while significantly increased the number of bacteria and actinomyce, number ratio of bacteria to fungi (B/F). Soil microbe diversity index also significantly increased under the three rotation systems. The effect of the watermelon → pepper treatment was good, under which the diversity index (McIntosh index) was 0.247, (vs CK of 0.146), proportion of actinomyce was 31.98% (vs CK of 14.22%), proportion of fungi was 0.06% (vs CK of 0.43%). Different crops had different water and nutrient requirements which resulted in different soil microbial populations. It was concluded that crop rotation changed soil microenvironment in gravel-sand mulched fields, increased diversity index and limited adverse effects of continuous cropping on watermelon. An efficient rotation system for resolving the obstacles in continuous cropping of watermelon was watermelon and pepper rotation system.