Reducing yarn hairiness is the greatest ascendancy for compact spinning. Based on the test of fiber length distribution, the hairiness of compact yarn is simulated in order to study the rule of hairiness producing. The hairiness of different fineness yarn is simulated by changing the twist factory and the diameter of yarn and the hairiness of different kinds of yarns such as compact yarn and ring spinning yarn are simulated by changing the transition probability of fiber. The simulation data are in accordance with the experimental data. It can make prediction about yarn hairiness according to the fiber length distribution in order to guide production.