INTRODUCTION: Paraneoplastic syndromes are common in cancers such as lung, breast, and ovarian cancers. Still, the dual paraneoplastic syndromes of ectopic adrenocorticotropic hormone (ACTH) syndrome (EAS) and raised pancreatic enzymes at the same time are rare. EAS is due to the production of ACTH by tumors other than the pituitary gland, which stimulates the hyperplasia of the adrenal cortex to secrete excessive corticosteroids, most commonly in lung cancer. Elevated pancreatic enzymes are associated with ectopic secretion from lung cancer. Clinically, some patients with small cell lung cancer (SCLC) have atypical early clinical manifestations and may present with paraneoplastic syndrome as the first symptom. CASE REPORT: This article describes a case of a 45-year-old male patient who was admitted to the hospital with "intermittent mild edema of both lower extremities for more than 1 month" and showed persistent low potassium without diuretic drugs and with abnormally high blood amylase and blood lipase in the exclusion of pancreatitis. The persistent low potassium was caused by unusually high cortisol levels in patients with EAS that result from large amounts of cortisol secretion. Pancreatitis was excluded, and he was finally diagnosed with extensive-stage SCLC after bronchoscopic biopsy and histopathological confirmation. The patient presented with dual paraneoplastic syndromes of SCLC combined with EAS, high pancreatic enzymes, dual metastases, high malignancy, loss of surgical opportunities, and poor prognosis. The patient died at the end of the first cycle of chemotherapy due to the combination of IV degree of myelosuppression, metabolic alkalosis, severe infection, respiratory failure, and the rapid deterioration of his condition. CONCLUSION: Most of the clinical manifestations of lung cancer with paraneoplastic syndrome as the first symptom lack specificity. The paraneoplastic syndrome of lung cancer can appear in all stages of the disease, and if it appears before the diagnosis of lung cancer, it is of some significance in guiding the diagnosis of lung cancer. Meanwhile, when pancreatic lesions are excluded, we should consider malignancy-related hyperpancreatinemia.