An image decoding method performed by a decoding device according to the present document comprises: a step for deriving the number of samples of upper peripheral chroma samples and left peripheral chroma samples of a current chroma block on the basis of a specific value and the width and height of the current chroma block; a step for deriving said number of upper peripheral chroma samples and said number of left peripheral chroma samples; and a step for deriving CCLM parameters on the basis of the upper peripheral chroma samples, the left peripheral chroma samples, and down-sampled peripheral luma samples, wherein, when the specific value is derived as 2 and the width and the height of the current chroma block are larger than the specific value, the number of samples is derived as the specific value.