A water quality sensor includes a housing having two hollow protruding portions, and a sensing module including a circuit board mounted inside the housing and having two positioning plates respectively positioned in the two hollow protruding portions, two identical connectors respectively mounted on the two positioning plates of the circuit board in reversed directions with a phase difference of 180 degrees therebetween, and a light emitter and a light receiver respectively mounted in the connectors to face toward each other for water quality detection. Thus, the invention allows the implementation of automated assembly to replace manual assembly, reducing the risk of human error, saving much labor and production costs, improving product quality and increasing product yield.