Objective To investigate the effect of serum containing Xinfeng capsule (XFC) on the angiogenesis of human umbilical vein endothelial cells (HUVEC) induced by rheumatoid arthritis fibroblast-like synoviocytes (RA-FLS) and its mechanism of action. Methods An in vitro co-culture model of RA-FLS and HUVEC was established. Serum containing XFC was prepared by oral gavage of SD rats. CCK-8 was used to screen the optimal co-culture ratio and XFC serum concentration. The lncRNA HOTAIR overexpression plasmid (pcDNA3.1-lncRNA HOTAIR), along with the negative control group, were constructed and transfected into RA-FLS. The experiments were done in HUVEC control group, model group (co-culture of HUVEC and RA-FLS), XFC group (co-culture of RA-FLS treated with 200 mL/L XFC), HOTAIR negative control group (co-culture of RA-FLS transfected with pcDNA3.1-NC), HOTAIR overexpression group (co-culture of RA-FLS transfected with pcDNA3.1-lncRNA HOTAIR), and XFC-treated HOTAIR overexpression group (co-culture of RA-FLS transfected with pcDNA3.1-lncRNA HOTAIR and treated with 200 mL/L XFC). The proliferation ability of HUVEC was detected by CCK-8 method. The migration ability of HUVEC was detected by TranswellTM method. The tube formation ability of HUVEC was detected by tubule formation assay. The expression of CD34 and CD105 in HUVEC was detected by flow cytometry. The expressions of lncRNA HOTAIR, miR-126-3p, phosphatidylinositol 3-kinase (PI3K), PI3K receptor 2 (PIK3R2), AKT, vascular endothelial growth factor (VEGF), and basic fibroblast growth factor (bFGF) mRNA in HUVEC were detected by real-time quantitative PCR. The protein expressions of PI3K, AKT, p-AKT, VEGF, and bFGF in HUVEC were detected by Western blot and immunofluorescence technique. Results The results of CCK-8 method showed that the optimal treatment ratio and time of RA-FLS and HUVEC co-culture were 5:1 and 48 h respectively. The optimal intervention concentration and time of XFC were 200 mL/L and 48 h. Compared with the control group, the proliferation, migration, tube-forming ability and CD34 and CD105 levels of HUVEC in the model group were significantly improved, the expressions of lncRNA HOTAIR, PIK3R2, VEGF, bFGF, PI3K, AKT and p-AKT were significantly upregulated, and miR-126-3p was significantly downregulated. Compared with the model group, the proliferation, migration, tube-forming ability and CD34 and CD105 levels of HUVEC in the XFC group were significantly decreased, the expressions of lncRNA HOTAIR, PIK3R2, VEGF, bFGF, PI3K, AKT and p-AKT were significantly downregulated, while the expression of miR-126-3p was significantly upregulated. Compared with the HOTAIR negative control group, in the HOTAIR overexpression group, the proliferation, migration, tube-forming ability and CD34 and CD105 levels of HUVECs were significantly increased, the expressions of lncRNA HOTAIR, PIK3R2, VEGF, bFGF, PI3K, AKT and p-AKT were significantly upregulated, and the expression of miR-126-3p was significantly downregulated. Compared with the HOTAIR overexpression group, the proliferation, migration, tube-forming ability and CD34 and CD105 levels of HUVECs in the HOTAIR overexpression group treated with XFC were significantly downregulated, the expressions of lncRNA HOTAIR, PIK3R2, VEGF, bFGF, PI3K, AKT and p-AKT were significantly downregulated, and the expression of miR-126-3p was significantly upregulated. Conclusion XFC-containing serum may play a therapeutic role by inhibiting the expression of lncRNA HOTAIR/PI3K/AKT pathway, reducing the expression levels of VEGF and bFGF, and alleviating synovial angiogenesis induced by RA-FLS to exert therapeutic effect.