The invention discloses a downproof finishing agent and a finishing process thereof on a downproof fabric; the downproof finishing agent is a mixed aqueous solution of an acrylate-ammonia ester-organosilicone copolymerized modified polymer, an acrylate-ammonia ester-organosilicone copolymerized modified polymer, a ternary copolymerized silicone oil and a surfactant, wherein the content of the acrylate-ammonia ester-organosilicone copolymerized modified polymer is 30-80 g / L, the content of the ternary copolymerized silicone oil is 10-50 g / L, and the content of the surfactant is 1-5 g / L. The finishing process comprises the steps: firstly, preparing the downproof finishing agent, and then padding, drying, preshrinking, calendering and re-preshrinking, to obtain a finished fabric. After the fabric is padded in the finishing agent, fiber cracks are plugged through bonding, the fastness is increased, and the fabric handfeeling is improved; then through compounding with the ternary copolymerized silicone oil softener and the surfactant and the preshrinking, calendering and re-preshrinking processes, the fabric is guaranteed to have good downproof effect and also have a soft, matte and active fabric style.