Several studies have shown positive responses of parasitism to either host density or landscape complexity. Many semi-natural arable habitats (Grasslands, woodlands, and wetlands) have been shaped by centuries of farming in China and worldwide. Established parasitoid populations have been recognized to provide pest management due to their high rates of consumption and precedence in wheat fields. However, no previous experiments have manipulated host density in agricultural landscapes of various complexity. Here we report the results of a field experiment conducted to determine how agricultural landscape complexity affects cereal aphids and how host density and agricultural landscape complexity jointly affect the parasitism, hyper-parasitism, and species diversity of parasitoids. Parasitism and hyper-parasitism were assessed by experimentally adding cereal aphids at low and high densities to commercial wheat fields located in two different agricultural landscapes in order to detect parasitism therein. Results showed that landscape complexity did not influence population density in cereal aphids. Landscape structure also did not influence parasitism and hyper-parasitism, contrary to our expectations. Increased host densities caused a trend of increasing abundance and species diversity in parasitoids and hyperparasitoids. Aphidius avenae Haliday parasitized a significantly greater proportion of hosts at low-host densities, while the opposite effect occurred for Aphidius gifuensis Ashmead. The discrepancy in responses of these parasitoids and hyper-parasitoids to host density is discussed in relation to differences in morphological traits, body size, and historical characteristics. The specific composition of parasitoid and hyper-parasitoid assemblage could significantly alter parasitism at various host densities at which landscape complexity and habitat composition impart negligible influence. Future studies should both evaluate plants with a more diverse range of traits and behaviors in landscapes with non-crop herbivore densities and span over more years to fully understand the impact of agricultural landscapes on species diversity and biological control service of parasitoids.