The invention provides an acid copper plating brightness stabilizer and a preparing method thereof. The acid copper plating brightness stabilizer comprises the following components in parts by mass: 3-7 parts of 3-(amidinothio)-1-propanesulfonic acid, 1-7 parts of a polyamide cross-linking agent, 0.1-0.2 part of polyethylene glycol and 84-95.9 parts of a solvent. The acid copper plating bright stabilizer adopts special contents of special components, the components act in a cooperative manner, and the acid copper plating brightness stabilizer can keep a system stable in a long-term continuous acid copper plating technology and effectively improves copper plating flatness, brightness and quality. The acid copper plating brightness stabilizer is obtained through adding the 3-(amidinothio)-1-propanesulfonic acid, the polyamide cross-linking agent and the polyethylene glycol into the solvent, uniformly mixing, agitating, dissolving and dispersing. The preparing method of the acid copper plating brightness stabilizer is simple, is low in energy consumption and is suitable for large-scale production.