A 40-day-old baby with heat rash can use prickly heat powder for topical application, and control the indoor temperature and wash regularly. Prickly heat is mainly caused by the blockage of sweat ducts. The development of sweat ducts in newborn babies is not yet complete, and the increase in the temperature of the surrounding environment induces the occurrence of prickly heat. The first thing to do is to keep the affected area clean and cool, take regular baths, then you can use prickly heat powder and glycerite lotion to apply on the affected area to cool and relieve itching. If the number of heat rashes is large and the itching is heavy you can use mild to moderate glucocorticoid ointment such as hydrocortisone under doctor’s supervision to alleviate the symptoms. In addition, it is important to keep the indoor temperature suitable, ventilate the room regularly, take off the baby’s extra clothes, try to wear loose and breathable cotton clothes, reduce the friction on the skin, and keep the skin clean and dry. Please follow your doctor’s instructions if you need to use medication.