When measuring infrared radiation characteristics, it is necessary to judge whether the target is a small target or a surface target. This paper proposes a judging method based on the effective imaging pixels of the target that takes into account the influence of the diffusing effect of the imaging system as well as that of the signal-to-noise ratio on the imaging process. The method employs a discrete imaging process model to simulate images of small targets and implements an algorithm for estimating the effective imaging pixels. Characteristic curves for judgment are obtained, and judgment criteria are, in turn, established based on the definition of a threshold number of effective imaging pixels. Targets with a number of effective imaging pixels less than this threshold are defined as small targets. Experimental results validate the feasibility and effectiveness of the proposed method.