To study the effects of different initial free airspaces (Fas) on pig slurry-wheat straw reactor scale composting, three levels of Fas (56.70%，62.67%，68.36%) were obtained by mixing pig slurry and wheat straw with the same mass ratio (1∶0.086) but in different wheat straw sizes (0~1cm, 2~5cm, 7cm). Oxygen concentration and temperature were monitored by sensors during the whole composting period, while moisture and volatile solid (VS) content were checked by chemical analysis method at the end of composting. In addition, a Monod form model was employed to simulate VS content during composting period. According to the results of composting test, changes of oxygen concentration, temperature, moisture and VS based on different initial Fas levels were respectively clearly different; effects of bulking agent sizes on mixture structure were apparent.