Porcine skeletal muscle development is closely linked to meat production efficiency and quality. The accumulation of porcine intramuscular fat is influenced by the hyperplasia and hypertrophy of adipocytes within the muscle. However, the cellular profiles corresponding to the two stages of muscle development remain undetermined. Single-nucleus RNA sequencing (snRNA-seq) can elucidate cell subsets in tissues, capture gene expression at the individual cell level, and provide innovative perspectives for studying muscle and intramuscular fat formation. In this study, a total of 78,302 nuclei and 9 clusters of cells, which included fibro/adipogenic progenitor (FAP), myonuclei, adipocytes, and other cell types, of Xidu black pigs, were identified on Day 1 and Day 180. The pattern of cell clustering varied between the two developmental stages. Notably, the percentage of adipocytes in the Day 180 group was higher than in the Day 1 group (0.51% vs. 0.15%). Pseudo-time sequence analysis indicated that FAPs could differentiate into adipocytes and myonuclei cells, respectively. The THRSP gene was identified as a biomarker for swine intramuscular fat cells, and its down-regulation resulted in significant reduction in lipid droplet formation in porcine preadipocytes. Our research provides new insights into the cellular characteristics of intramuscular fat formation, which may facilitate the development of novel strategies to enhance intramuscular fat deposition and improve pork quality.