The most common modulation transfer function (MTF) measurement equipment operates along a coaxial testing light path. It can be used to test several fields along one radial direction and is suitable for an optical system with rotational symmetry. However, off-axis optical systems need multidimensional adjustment and complex mechanical structures to measure the MTF of fields. In this paper, we propose a MTF testing module to address this issue by adding a rotatable mirror to redirect the light. The testing module greatly simplifies MTF measurement of off-axis imaging systems in both the process and mechanism. The figure error of the rotatable mirror is analyzed to ensure testing accuracy. MTF testing of a free-form surface prism using this novel method was successfully implemented and the results are presented. The method can be extended to the measurement of other parameters, such as the effective focal length.