A method for infrared (IR) small dim target detection based on a local contrast mechanism is proposed to solve the problem of IR small dim target detection under a complex background and low signal-to-clutter ratio (SCR). A three-layer window consisting of an inner layer, a middle layer, and an outer layer is proposed, so that targets of different scales can be detected using only single-scale calculations. First, the matched filter is applied to the inner layer to enhance the true target purposefully, and the max-close principle is proposed to estimate the background of the outer layer, so that detection becomes easier when the target is near the background edge. Then, the ratio-difference joint local contrast measure is calculated between the enhanced target and the estimated background to enhance the true target and suppress the complex background simultaneously. Finally, an adaptive threshold operation is used to extract the true target. Experimental results show that compared to some existing algorithms, the proposed algorithm can enhance the true target and suppress complex background better, and its principle is simple yet suitable for implementation and can effectively reduce the amount of calculation.