Interval estimation for asynchronous switched systems is investigated on the basis of zonotope. A particular zonotopic estimation framework that includes center estimation and P-radius estimation is presented. In the light of robust stability theory, an asynchronous robust observer, which is used in the center estimation, is established. And the P-radius estimation is conducted by zonotopic set-membership estimation theory. The proposed framework reduces the conservatism that exists in interval observer design methods. Besides, to maintain a high accuracy, the direct method that disregards order reduction is applied instead of the iterative method in this framework. Finally, the simulation of a missile flight dynamics system is carried out to illustrate the validity of the estimation framework.