Interspecific competition among different species of insect parasitoids may affect the establishment or efficacy of species used for biological control of a target pests. Peristenus spretus Chen & van Achterberg and Peristenus relictus Loan (syn. P. stygicus) (Hymenoptera: Braconidae) are two dominant solitary parasitoids of mirid bugs in China. In this study, interspecific competition between larvae of P. spretus and P. relictus for the nutrients inside their common host, the green plant bug (Apolygus lucorum Meyer-Dürin), as well as their extrinsic competition for host resources were investigated in the laboratory. We examined the effect of four different time intervals between oviposition by the two species (10 min, 24 h, 48 h, and 72 h) and two parasitoid sequences (i.e., P. spretus before P. relictus [Ps-Pr] and P. relictus before P. spretus (Pr-Ps)] in no-choice tests to measure nutrient competition between the two species. In choice tests, three densities of A. lucorum nymphs (40, 80, and 120) were presented to various combinations of P. spretus and P. relictus (1,1; 2,2; or 3,3 pairs). In addition, the developmental times of the immature stage (from egg to late instar larva) of P. spretus and P. relictus were determined. We found that neither the time interval between oviposition by the two competing species, nor the order the parasitoids attacked (Ps-Pr or Pr-Ps) had any influence on the percentage of parasitoid offspring that belonged a particular parasitoid species. Peristenus spretus dominated, accounting for 89.2–97.9% of parasitoid offspring in the no-choice nutrient competition experiment. Parasitoid sequence did, however, affect sex ratio in the dominant parasitoid’s offspring, with 48.2% of P. spretus offspring being female in the Pr-Ps sequence, while only 29.2% were female in the Ps-Pr sequence. Overall, in the host resource competition experiment, 64.7–82.2% of all emerged adult parasitoids of the offspring generation, for all host density treatments, were P. spretus. The percentage of emerged parasitoids that were a given species was significantly affected by host density, but neither by the number of pairs of P. spretus or P. relictus nor by the parasitoid density × host density interaction. The developmental time of the immature stage of P. spretus was 2.2 days shorter than that of P. relictus, an important factor in making P. spretus a superior competitor over P. relictus in all the competition trials. Based on these results, we conclude P. spretus will have an important advantage when competing with P. relictus, a factor that should be considered in any biological control program using these two parasitoids in China.