A telescopic tube with water supply is equipped with a minor tube, a major tube that are connected to each other by a tubular coupling device which has a registration end and a connecting section, the former is engaged with one end of the minor tube and the latter is engaged with the top end of the major tube. A guard cap is secured to the top end of the major tube which serves to prevent the minor tube from easy disengagement from the major tube in case of excessive extension adjustment. The connecting section of the tubular coupling device has a cone shaped configuration with a spiral thread track defined thereon and terminates in a stop board. A pair of support halves having tapered inner surface are movably registered with the spiral thread track and are outwardly expanded or retracted as moving against the cone shaped connecting section so that the outer surface of the support halves can be urged against the inner side of the major tube to hold the same in place in length adjustment and vice versa for retraction of the telescopic tube. Thus the telescopic tube can be adjusted of its length and secured in place by simple rotation of the major tube and moving the minor tube to make the adjustment easy, speedy and precise.