Cropland area has long been used as a key indicator of food security. However, grain yield is not solely controlled by the area of the cropland. Therefore, we proposed a new indicator to assess food security. Results show that from 1992 to 2004, the global cropland area increased by 840 200 km(2) (99.4%), but the grain yield increased only by 310 million t (29.1%); and from 2004 to 2015, the cropland area decreased by 39 000 km(2) (4.64%), but the grain yield increased by 370 million t (70.84%). This result showed that grain yield was not linearly correlated with cropland area, and delimiting the threshold of cropland protection may not guarantee food security. Combined with further correlation analysis, we found that the increase in the global grain yield was more closely related to the harvested area (R-2 = 0.94), which indicated that the harvested area is a more scientific and accurate indicator than cropland area in terms of guaranteeing food security. Therefore, if governments want to ensure the food security, they should choose a new and more accurate indicator: harvested area rather than cropland area.