The cold and dry winter in the north is most suitable for swimming, and its fitness value is higher than that in summer. And the feeling of swimming in winter is different from that in summer. Swimming in winter is generally from outdoor to indoor. In this way, on the one hand, it can be free from outdoor air pollution; On the other hand, when swimming, due to the stimulation of cold water on the skin, the blood vessels of the skin contract sharply, and a large amount of blood is driven into internal organs and deep tissues. After the blood vessels contract strongly, we must relax accordingly and let the blood vessels get exercise one by one. So that the human body can adapt to this alternating change of cold and heat more quickly. Swimming in winter can also effectively improve the immune ability of human body to a certain extent, which can make you resist the flu in spring and summer, which is the best proof that swimmers are not easy to catch a cold. At the same time, the swimmer is in a horizontal state, and the heart and lower limbs are in the same plane, so that when blood flows back from the great vein to the atrium, it is not necessary to overcome the force, which creates favorable conditions for blood circulation. The ventricle is filled with blood returning to the heart, which is beneficial to improve the function of cardiovascular system. In addition, the flow of water and the movement of muscles play a role in massaging arterioles. This regular massage can reduce the hardening of arterioles and reduce the peripheral resistance encountered when the heart ejects blood. It can prevent hypertension and heart disease.