Introduction: Quadrilateral space syndrome (QSS) is a peripheral nerve entrapment disease, which can be misdiagnosed in clinic. In the past, QSS was mainly diagnosed by clinical symptoms combined with magnetic resonance imaging (MRI), electromyography (EMG), and arterial angiography. There are few reports on the diagnosis of QSS by musculoskeletal ultrasound (MSKUS) combined with clinical symptoms. Patient concerns: A middle-aged female patient had posterolateral pain and numbness in her right shoulder for 2 months. Diagnoses: At first, she was diagnosed as suprascapular nerve entrapment, while EMG of suprascapular nerve and axillary nerve indicated that nerve conduction was normal. Then, MRI was performed, showing the shoulder had no abnormalities, and EMG and arterial angiography of upper limb showed no abnormalities too. Finally, she was diagnosed as QSS according to MSKUS and lidocaine block test. Interventions: Two sealing treatments of axillary nerve block in quadrilateral space under the guidance of MSKUS were performed. Outcomes: After 2 treatments, the pain and numbness in her shoulder disappeared, and her shoulder could move normally. There was no recurrence after 3 months of follow-up. Conclusion: MSKUS is an effective method to diagnose QSS. It is fast, convenient and inexpensive, and is worth popularizing in clinic.