Habitat degradation changes available food resources for lizards in their environment. We studied the diet of the toad-headed lizard (Phrynocephalus frontalis) through stomach content analysis. Lizards were collected in Jungar Banner and Dalad Banner in the Erdos region of Inner Mongolia, China. Experiments identified and analyzed 652 suits of food in 77 stomach samples from three fields including a fixed dune, semi-fixed dune, and mobile dune. Food primarily consisted of 35 groups of arthropods belonging to 3 classes, 11 orders, and 32 subjects. In addition, we also found a small amount of plant debris (1.95%) in the stomach contents of a few individuals (4.69%). Among the three dune habitats, Formicidae were the most important items in the diet, followed by Aphididae, Coccinellidae, Cicadellidae, and Braconidae. However, toad-headed lizards changed food preferences more frequently in the semi-fixed and mobile dunes compared to food in the fixed dune habitat, leading to a large food dissimilarity between the mobile dune and the other two habitats. With increasing desertification of habitats from the fixed dunes, semi-fixed dunes, and mobile dunes, the abundance of prey groups for toad-headed lizards is decreasing, and the Simpson's dominance index fell accordingly. However, the niche breadth, the Shannon-Weaver diversity index, and the Pielou's evenness index significantly increased in the mobile dune habitat. This result shows toad-headed lizard in the Erdos region is an omnivorous animal and mainly preys upon insects and eats a very small number of plants. Habitat desertification significantly affected food composition of the toad-headed lizard, and gradually reduced the type of foraged food.