Abstract: To increase the efficiency of airspace use, an entry clearance strategy of the crossing air route was proposed and a mathematical model was established. Then, a new method for evaluating the airspace time-slot availability, with different combinations of various crossing route structure patterns and entry clearance strategies, was provided. The influence on the time-slot availability of the crossing air route imposed by the structural constraints, such as route length and crossing angle, and the deviation angle were analyzed by simulation. The results show that extending the route length under the fixed deviation angles could enhance the airspace time-slot available rate to a maximum level as high as 82%, 90%, and 97% when the deviation angle is 10°, 20°, and 30°, respectively. However, the approach of extending route length has a marginal effect; that is, there is no more promotion effect in the time-slot availability when the route length exceeds a threshold. Under the same route length and deviation angle, increasing the crossing angle would also enhance the airspace time-slot availability.