Erythema multiforme is an acute inflammatory skin disorder with a polymorphic rash that may be accompanied by mucosal damage. In severe cases, there is severe mucosal and visceral damage. The lesions are polymorphic, occurring in spring and autumn, and are more common in young women. The diagnosis needs to be made based on the test results. Diagnosis is aided by routine blood sampling (blood rheology is an eight-index test that reflects the consistency, viscosity, plasma viscosity, aggregation and coagulation of blood cells), urine testing, and chemical testing of skin surface bacteria. Patients with rashes commonly have significant blood flow changes. Some patients with genetic or immunologic abnormalities, such as psoriasis, have more significant alterations in blood rheology. Therefore, the detection of blood rheology in patients with psoriasis presents a reliable basis for the occurrence, development, regression, and prognosis of psoriasis, and similarly, patients with erythema multiforme-like rash have great clinical significance for the subsequent development, regression, and prognosis of the disease through clinical examination.