Accurately measuring the focal length of an infrared lens is a crucial issue in infrared lens parameter measurement. This paper proposes a practical focal length estimation method based on a target knife image. First, the target image is captured in the focused state and a binary image is obtained, then the edge of the target is extracted, the vertex coordinates of the minimum external rectangle are obtained, and the corresponding focal length of the infrared lens is estimated. Experimental results show that the algorithm can measure the focal length of the infrared lens quickly and accurately, and the average absolute error percentage is less than 1.48. This method lays the foundation for the rapid measurement of infrared lens parameters.