With the increasing demand for sustainable animal production, the utilization of agricultural and processing by-products as functional feed ingredients has gained growing attention. However, the application of Xiasangju processing residues, a by-product generated during the industrial processing of the traditional Chinese herbal formula Xiasangju, in poultry nutrition remains poorly understood. This study aimed to evaluate the nutritional composition, bioactive components, and potential functional effects of Xiasangju processing by-products as a dietary supplement for late-laying hens. Chemical composition was characterized using LC-MS and conventional nutritional analysis, while potential anti-inflammatory mechanisms were predicted via network pharmacology. A total of 288 Jingfen laying hens (55 weeks old) were randomly assigned to four dietary treatments supplemented with 0, 0.5%, 1.0%, or 1.5% Xiasangju processing by-products for 56 days. Results showed that 11 major active compounds were identified, including relatively high levels of linarin and rosmarinic acid, along with abundant crude protein, fiber, minerals, and amino acids. Dietary supplementation at 1.5% was associated with higher egg production rate, egg weight, and yolk color without obvious adverse effects on organ indices or serum biochemical parameters. This treatment was also associated with lower inflammatory gene expression, including IL-6 and COX-2 in the ileum and NF-κB, IL-6, COX-2, and TNF-α in the cecum. No significant effects were observed on antioxidant status in yolk or liver, or intestinal morphology. Integrated analyses using network pharmacology, molecular docking, and in vitro COX-2 inhibition suggested that anti-inflammatory-related responses may involve COX-2-related pathways, while 16S rRNA sequencing indicated changes in gut microbiota composition. In conclusion, Xiasangju processing by-products may serve as a potential functional feed ingredient for late-laying hens, with 1.5% inclusion showing favorable overall performance under the conditions of this study. These effects may be associated with the combined contribution of residual nutrients and bioactive compounds in the residues.