Small-scale self-focusing is an important factor threating the safe operation of intense laser facility, and estimating the marginal value of small-scale self-focusing in laser beam is an important part of the guidance for the safe operation of intense laser facility. The method of using near-field intensity entropy of the output laser beam to reflect the small-scale self-focusing development in laser beam is proposed. Then the variation regular pattern of near-field intensity entropy with the development of small-scale self-focusing is studied by numerical simulation. Using this regular pattern, an early-warning method is given for the obvious small-scale self-focusing happening in intense laser facility. By contrast with the methods of modulation ratio and contrast ratio, the proposed method offers a more obvious assessment.