In order to illustrate the competitiveness of three different ploidy wheats(the diploid Triticum monococcum, the tetraploid Triticum dicoccum,and the hexaploid Triticum aestivum Changwu 134), field experiment was conducted in Yangling with three planting densities (low,medium and high density, exactly 1, 2.5 and 4 million plants per hectare), two kinds of water conditions were taken as well respectively one with normal rainfall and irrigation while the other with none. The response of plant height, above-ground biomass, the tiller number per plant, grain yield and yield components to the density and water conditions among three wheat varieties was investigated. The result indicates that: 1) As wheat population density increased, the competition between individuals turned more intensive, biomass and tiller was obvious reduced, biomass accumulation rate declined. On the other side, the ample supply of moisture promoted the individual growth, difference between populations increased significantly along with individuals competition decreased. The diploid and tetraploid got the greatest height under medium density at harvest time,comparing with hexaploid what; 2) With the increase of population, the competition was strengthened and individual fecundity decreased gradually. Although spike density increased significantly, the strengthening of individual competition for resources limited the dry matter accumulation in post flowering, ultimately resulting in a significant declined in individual reproductivty, the final induced that the grain yeild did not increase with the population increased, especially under water deficit. The results showed that there is not a determined positive linear relationship between population size and yield. Under normal water supply both tetraploid and hexaploid got the highest yield under low density, which proved that high yield could be obtained under appropriate density in the premise of water saving with reasonable water strategy, improved population photosynthetic capacity, increased dry matter accumulation and proportion in post flowering;3) Tiller number and plant height of Changwu 134 was lower than diploid and tetraploid, the individual ability to compete for resources was lower, while the biomass accumulation rate had obvious superiority. Tetraploid showed the strongest ability in individual competition among the three ploidy wheats. Chagnwu 134 abtained the highest population yield under both drought and irrigation treatments, the two primitive species showed no significant difference meanwhile. Chagnwu 134 put up no obvious superiority on grain number per spike and spike density, but seed weight reached around 50 g/per 1000, and harvest index was close to 50%. Modern variety Chagnwu134 proved the the most significant yield reduction under drought condition, and showed up the strongest response to density under drought stress, which confirmed that drought influenced it most and it had a lowest drought resistance ability.The low competitive ability is the foundation in high population production. Modern wheat varieties put up more reasonable dry matter allocation ability, and they are more close to ideal plant type, which means that modern wheat variety receive high yield from the decrease of individual competiveness. All above-mentioned results provides the theory basis and good understand for water saving and high yield cultivation and the breeding in the arid and semi-arid area.