Abstract: Abstract: As the producer of compressed air, portable twin-screw air compressors are widely used in many industrial applications taking into account its reliability and compact feature. The compressor system is frequently operated under unload condition to achieve the air flow consumption. Under the unload condition, the air compressor still requires a lot of energy, but the air compressor stops supplying compressed air to the network. This is often ignored when studying the energy consumption of the compressor. It is important to research the dynamic performance of the portable twin-screw air compressor to reduce the energy consumption of compressor system. The system composition of portable twin-screw air compressor was described and the operating principle was analyzed. The twin-screw air compressor was divided into 3 subsystems: diesel engine, twin-screw compressor and intake valve control. For each subsystem a mathematical model was established and explained in detail. The dynamic simulation model of the portable twin-screw air compressor was implemented on the platform of MATLAB/Simulink. Three different air flow consumption loading patterns of compressor were simulated in order to understand the dynamic performance of the portable twin-screw air compressor with different air flow consumption. Some primary performance parameters of the screw compressor under load and unload conditions, such as system pressure, flow rate and shaft power, were analyzed by numerical simulations. Experimental investigation on a twin-screw air compressor under load and unload conditions was carried out to verify the integrated model. The performances of the compressor under load/unload conditions, such as system pressure and shaft power consumption, were measured with the experimental apparatus. The research results showed that the dynamic characteristics of twin-screw air compressor system under load/unload conditions were influenced by air flow consumption. According to the influence of air flow consumption on the screw compressor system, the system pressure had obvious fluctuation with the variable air flow demand pattern. The pressure change rate decreased with the increased air flow consumption. It meant that the load/unload frequency of screw compressor was reduced. When the air flow consumption was greater, the twin-screw air compressor had a greater average shaft power of system, a smaller shaft power of air flow consumption per unit and a higher energy utilization ratio. When the fluctuation range of air flow consumption was greater, the twin-screw air compressor had a greater average shaft power of system, a greater shaft power of air flow consumption per unit and a lower energy utilization ratio. Therefore, when the twin-screw air compressor runs under the load/unload conditions, it should reduce the fluctuation range of air flow consumption, and increase the air flow consumption to improve working efficiency. Simulation results are in good agreement with the experimental ones. It was shown that the error between the measured and calculated data was less than 5%. So the dynamic working process of twin-screw compressor under load/unload conditions can be predicted exactly by means of the mathematical models. Therefore, this research can be useful for energy-saving running of twin-screw air compressors.