The invention discloses an arc fine adjusting method of a slab continuous caster, belonging to the technical field of slab continuous casting. The method comprises the following steps of: finding out a relationship between spacing piece quantity added on or deducted from a fan-shaped section support seat and arc change by means of a CAD simulation method, and detecting an original data; computingto obtain an adjusting quantity according the actually detected original data and a solving program; and computing according to the actually detected original data and the solving program to comprehensively analyze theoretical results, and confirming the final adding and deducting quantity of the spacing piece by taking smooth transition of the arc section, theoretic gap value and least adjustingpart as a predominant idea. The method has the following advantages of greatly improving arc adjusting efficiency and arc adjusting precision due to the methods such as the CAD simulation technique, matrix, programming, and the like, in the process of detecting and adjusting.