The optical transfer function is accepted as an essential index to evaluate the imaging quality and is an important auxiliary parameter for designing the optical system. Based on the background oriented schlieren, the displacement field data of the light passing through the refractive index field are obtained, and the corresponding wavefront gradient data is obtained by combining the Marius theorem. Combined with the geometric optical transfer function formula, the optical transfer function can be achieved. The optical transfer function of a flat convex lens is measured by the above idea and is compared with the optical transfer function results calculated by ZEMAX. Comparison of the experimental results and theoretical results at low frequencies is conducted and the error is less than 4%. Feasibility and validity of background oriented schlieren for measuring the optical transfer function of low-pass systems are verified.