A Canny edge detector by scale multiplication is proposed in infrared image processing, a scale multiplication function is defined as the product of the responses of the detection filter at two scales. Using the non-maximal suppression method, the edge map is obtained by confirming the gradient map of the scale multiplication. It can be demonstrated that the performance of the traditional Canny detection operator is improved through the experiment and the detection accuracy is better. A good tradeoff effect can be attained at the localization and the noise suppression.