The present invention discloses a water treatment agent composition, which contains an amino acid having a scale inhibiting and corrosion inhibiting effect, a macrolide antibiotic having a sterilization effect, and an enzyme cooperating with the amino acid and the antibiotic and providing a synergy effect. The present invention further discloses applications of the water treatment agent in water treatment. According to the present invention, the performance evaluation results of the water treatment agent show that the calcium carbonate scale inhibiting rate is more than 99%, the corrosion rate of the dynamic simulation test is 0.023 mm/a, and the number of the 72 h heterotrophic bacteria can minimally achieve 1.5*10<2>/ml; and the composite agent has characteristics of low use amount, no phosphorus, strong scale inhibiting capacity, good corrosion inhibiting performance, excellent sterilizing effect, and long lasting time.