The invention relates to a foaming-type heat-preservation method for a braze welding heat exchanger. The foaming-type heat-preservation method comprises the following steps that 1, a mold is opened for cleaning; 2, the inner wall of the mold is baked; 3, a release agent is coated; 4, a foaming agent is weighed and proportioned; 5, the heat exchanger is hoisted in the mold, and the foaming agent is poured in the mold; 4, the heat exchanger is taken out; 7, surface foaming layer treatment is conducted; and 8, a finished product is cleared and put in a storage. According to the foaming-type heat-preservation method for the braze welding heat exchanger, the foaming agent for surface modification through fluoroalkyl silane is adopted to wrap the heat exchanger, the whole product is wrapped with silver paper, heat loss inside the heat exchanger is avoided, and the heat exchanging efficiency is improved.