Highland barley is an important staple food in the Tibet, and the Tibetan Plateau is experiencing obvious climatic warming. However, few studies have examined the warming effects on highland barley growth and biomass allocation under conditions of controlled experimental warming. This limits our ability to predict how highland barley will change as the climate changes in the future. An experiment of field warming at two magnitudes was performed in a highland barley system of the Tibet beginning in late May, 2014. Infrared heaters were used to increase soil temperature. At the end of the warming experiment (September 14, 2014), plant growth parameters (plant height, basal diameter, shoot length and leaf number), biomass accumulation parameters (total biomass, root biomass, stem biomass, leaf biomass and spike biomass), and carbon and nitrogen concentration parameters (carbon concentration, nitrogen concentration, the ratio of carbon to nitrogen concentration in root, stem, leaf and spike) were sampled. The low- and high-level experimental warming significantly increased soil temperature by 1.52 and 1.98 °C, respectively. Average soil moisture was significantly decreased by 0.03 m3 m−3 under the high-level experimental warming, while soil moisture under the low-level experimental warming did not significantly change. The low- and high-level experimental warming did not significantly affect plant growth parameters, biomass accumulation parameters, and carbon and nitrogen concentration parameters. There were also no significant differences of plant growth parameters, biomass accumulation parameters, and carbon and nitrogen concentration parameters between the low- and high-level experimental warming. Our findings suggest that the response of highland barley growth, total and component biomass accumulation, and carbon and nitrogen concentration to warming did not linearly change with warming magnitude in the Tibet.