The invention discloses a simple and easy underwater flange installation machine, wherein the simple and easy underwater flange installation machine comprises a left hydraulic cylinder supporting frame (1), a right hydraulic cylinder supporting frame (2), a hydraulic cylinder (3), and a flange sliding rail (4), the piston rod end of the hydraulic cylinder (3) is fixed on the left hydraulic cylinder supporting frame (1), the cylinder barrel end of the hydraulic cylinder (3) is fixed on the right hydraulic cylinder supporting frame (2), one faces of the left hydraulic cylinder supporting frame (1) and the right hydraulic cylinder supporting frame (2), opposite to the left hydraulic cylinder supporting frame (1) and the right hydraulic cylinder supporting frame (2), are respectively provided with a left plug-in pin (11) and a right plug-in pin (21) which are opposite to each other, and the flange sliding rail (4) is groove-shaped. Compared with a conventional manual installation working relying on divers, working hours can be greatly shortened. Taking a pair of flanges as an example, a day is needed in a conventional method, but half of the time can be saved by utilizing the machine, and therefore not only is working efficiency improved, but also physical strength consumption of the divers are better relieved well. In addition, the simple and easy underwater flange installation machine has the advantages of being simple in structure, small and exquisite, low in cost, and strong in practicability and promotional value.