The windows are often used in the telescopes and large aperture collimators. A window is placed at the front of the system, and its transmitted wavefront error has great influence on the image quality of the system. The way of measurement determines the machining precision. When the window is greater than the current commercial plane interferometer with the maximum diameter of Φ800 mm, it′s unable to carry out the full aperture testing. To solve this problem, a new method for testing full aperture window transmission is put forward that the window is put in the optical system consisting of a spherical interferometer and a concave spherical mirror with big diameter and long focal length. The method has been successfully used to test a fused silica window, with Φ856 mm diameter and 35 mm thickness. The root mean square (RMS) value of measurement result is 0.0191λ(λ=632.8 nm), which satisfies the design requirements. The feasibility of this method used in transmission measurement of large aperture windows is verified.