Cassia fistula (C. fistula) has shown strong anti‐inflammatory, hepatoprotective, antitussive, antibacterial, and antifungal properties and is being used for healing wounds and gastrointestinal illness. This study was planned to obtain fruit extract from C. fistula using ultrasonic‐assisted extraction (UAE) technique and evaluated for phytochemical contents, anti‐hyperlipidemia, and hematological parameters. The results showed that total phenolic (TPC), total flavonoids (TFC), condensed tannin (CT), and saponins were 13.07 mg GAE/g, 5.24 mg QE/g, 4.01 mg/g, and 27.55%, respectively, in the extract. Proximate composition of the extract showed 2.48%, moisture, 1.25% fat, 2.80% ash, 4.59% fiber, 11.93% protein, and 76.95% NFE. The 2, 2‐diphenyl‐1‐picrylhydrazyl (DPPH) and ferric reducing antioxidant power assay (FRAP) activity was 63.30 μg/mL and 15.02 nmol/g, respectively. High‐fat diet (HFD)‐induced hyperlipidemic rats were orally administrated with 0.5 and 1.0 g of extract/kg body weight (bw) daily. The reduction of total cholesterol (TC: 90.83 ± 8.86 mg/dL), triglycerides (TG; 74.16 ± 9.10 mg/dL), and low‐density lipoproteins (LDL; 74.83 ± 4.66 mg/dL) and increase of high‐density lipoproteins (HDL; 41.83 ± 8.4 mg/dL) was observed. Significant changes in red blood cells (RBCs; 8.03 ± 0.67106/μL), mean corpuscular hemoglobin concentration (MCHC; 35.02 ± 1.78 g/dL), mean corpuscular hemoglobin (MCH; 18.00 ± 0.26 pg), and mean corpuscular volume (MCV; 55.36 ± 4.01 fL) at 1.0 g extract intake was observed. Extract administration also improved significantly liver enzymes, body weight, and liver morphology. Therefore, C. fistula extract can be effectively used as a therapeutic agent to improve serum biochemistry and hematological values.