We reported the diagnostic and therapeutic process of a young male patient with systemic lupus erythematosus (SLE) who presented with severe hyponatremia as the main manifestation upon admission, and analyzed and discussed the case. The patient was a 19-year-old young male with a subacute course of disease, fever ≥38.3 ℃ that could not be explained by other causes, acute and subacute cutaneous lupus erythematosus, oral ulcers, arthritis, leukopenia (< 4×10⁹/L), low C3+low C4, and positive anti-double-stranded DNA (anti-dsDNA). According to the 2019 American College of Rheumatology/European League Against Rheumatism (ACR/EULAR) classification criteria, the score was 27 points. The patient was admitted to the hospital with SLE. After admission, further diagnosis of lupus was confirmed, excluding infection, tumor, endocrine disease, etc. Hyponatremia was the main complication of this lupus patient. Hyponatremia was a rare complication of lupus, only a few cases have been reported. In this study, the paient ' s blood osmotic pressure was significantly reduced, which was considered to be hypotonic hyponatretic, urine osmotic pressure increased, maximum urine dilution caused by excessive water intake such as primary polydipsia, hypoosmotic fluid intake, and beer drinking were excluded, and 24 h urine volume and sodium were improved. The urinary sodium concentration was close to 20 mmol/L although with severe hyponatremia, considering the possibility of isovolemic hypotonic hyponatremia, the syndrome of improper secretion of antidiuretic hormone or adrenal cortical insufficiency. The patient had no manifestations, such as hypotension, typical site pigmentation, and high potassium, and there was little possibility of adrenal cortical insufficiency, and syndrome of inappropriate antidiuretic hormone secretion (SIADH) was considered for hyponatremia in the patient. The etiological mechanism of hyponatremia in lupus patients is not clear, but it is related to acute kidney injury, drugs and systemic inflammation. In this case, we reported for the first time that SLE was associated with abnormal hypothalamic signals, suggesting a possible mechanism of lupus hyponatremia. The patient underwent water restriction, intravenous and oral sodium supplementation, and the blood sodium quickly returned to normal after pulse therapy. The abnormal signal of the head magnetic resonance imaging (MRI) fornix column was improved after 1 month of treatment, further confirming our diagnosis. SLE complicated with hyponatremia is rare, but severe hyponatremia can be life-threatening, and attention should be paid to it. The possibility of neuropsychiatric lupus should be vigilant in patients with lupus combined with hyponatremia.