The invention discloses a waterproof after finishing method for a water-washable silk floss quilt, which comprises the following steps: (1) preparing a waterproof finishing agent; and (2) heating the waterproof finishing agent to between 35 and 45 DEG C, putting dry silk floss into the heated waterproof finishing agent for soaking for 20 to 30 minutes, then dehydrating, preliminarily drying and drying the soaked silk floss, and manufacturing the silk floss into silk floss quilt by using a conventional method, wherein the waterproof finishing agent comprises 2 to 6 percent of waterproof agentA, 0.6 to 3.6 percent of waterproof agent B, 0.5 to 5 percent of silk floss softening agent and 0.2 to 0.8 percent of silk floss swelling agent in percentage by weight, and water; the waterproof agent A is polymethyl hydrogen-containing siloxane; and the waterproof agent B is polydimethyl siloxane. The silk floss quilt prepared by the method achieves real water washing, and can keep the primary rebound resilience, bulky property and softness after five times of water washing.