A construction method of a water storage tank is provided to fix the location of underflow pipes by fixing brackets to the front and the rear of the outer circumference of a pair of underflow pipes and preventing the rotation of underflow pipes. A construction method of a water storage tank comprises the steps of providing a space under the ground and placing a pair of underflow pipes provided with a connecting hole(100), positioning the connecting hole formed on each underflow pipe to correspond to each other and fixing brackets provided with an arc-shaped groove to the front and the rear of the outer circumference of each underflow pipe(200), placing a connecting pipe between the connecting holes of each underflow pipe and welding the underflow pipes(300), sealing up the front and the rear of each underflow pipe with a front plate and a rear plate to complete a water storage tank(400) for storing rainwater, and filling up the outside of each water storage tank and the space under the ground with soil(500).
