The small bumps on the arms may be caused by eczema, keratosis pilaris and tinea corporis. 1. eczema: its cause is not yet clear, may be systemic diseases, genetic, immune abnormalities and other factors, the clinical manifestations of erythema, corn-sized papules and papules and other symptoms, commonly found on the head and face, hands and feet and limbs and other places. 2. Keratosis pilaris: its cause is not yet clear, may be genetic, metabolic disorders and vitamin A deficiency and other factors. Clinical manifestations of small papules, mostly light red, normal skin color, chicken skin-like, preferably in the upper arms or thighs and other places. 3. Tinea corporis: lesions are mainly papules, papules and red scaly patches, commonly found on the trunk, arms and other places, often accompanied by itching. The cause of the disease is mostly related to red trichophyton and other dermatophyte infections. There are many reasons that lead to small bumps on the arms, and patients are advised to seek medical examination to clarify the causes.