The invention relates to a central element of a reverse osmosis separating device, a reverse osmosis separating device and a manufacturing method. The central element comprises an external discharge pipe and a porous internal discharge pipe, wherein the external discharge pipe is provided with an internal space and an axial gap; the gap starts from the first end of the external discharge pipe, and extends towards the second end of the external discharge pipe; the porous internal discharge pipe comprises a first part and a second part; the first part can be placed into the internal space of the external discharge pipe; the second part can be pressed against the second end of the external discharge pipe, and is used for sealing the second end; the external discharge pipe can be used for accommodating the first part of a permeable membrane assembly into the internal space of the external discharge pipe, and placing the second part of the permeable membrane assembly on the outer surface of the external discharge pipe to form a multilayer membrane assembly; the gap can be used for accommodating a connecting part between the first part and the second part of the permeable membrane assembly; and the first part of the porous internal discharge pipe can be placed into the first part of the permeable membrane assembly.