Grassy strip is the main area for species migration, community succession, matter and energy exchange among patches of farmland system, which is of great importance for maintaining agricultural biodiversity and improving farmland productivity. Based on the crop-pest-natural enemy interactions, grassy strips with different widths (2 m, 4 m, 6 m) and their neighboring fields as well as the crop strips with the same widths were selected. The samples of spider and carabid were collected by the method of pitfall traps. Then we investigated the effects of grassy strip width on species population, movement rules, and the potential of pest control in farmland. The results showed that the number of spider and carabid individuals in grassy strip was significantly higher than that in crop strip, the abundance of spiders and carabids species in 4 m and 6 m grassy strip was significantly higher than that in 2 m grassy strip, and the species richness and abundance in grassy strip were significantly higher than that in crop strip. In addition, there was high heterogeneity of spider and carabid communities in different widths of the grassy strip. In 4 m and 6 m grassy strips and their adjacent maize fields, the abundance of spiders and carabids was significantly higher in grassy strip and the junction between the grassy strip and their adjacent maize field than that in the fields. With the increase of distance to grassy strip, the spider species abundance showed a significant decrease trend. The width of the grassy strip and the distance jointly determined the spatial distribution of spiders and carabids. The results of Monte Carlo test showed that both the width of grassy strips and the Margalef index of vegetation had a significant impact on the community structure of spiders and carabids. Meanwhile, both the dominant families of spiders and the dominant species of carabids were distributed in the corn field adjacent to 6 m grassy strip. The ratios of spider and carabid to maize pests were significantly higher in 4 m and 6 m grassy strip than that in 2 m grassy strip. The maize field with 4 m and 6 m grassy strip had significantly increased the hundred-grain weight of maize grains and the total yield of maize compared with common maize field. The study indicates that the grassy strip is of great importance for conserving predatory natural enemies to prevent pest occurrence in maize field. Taking into account the field occupation, conservation of natural enemy and biological control of pests, we suggest that the grassy strip of 4 m width is the most potential farmland boundary model for popularization.